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  This is a video tutorial on the basics of how to use the raw filtered images
 supplied by NASA's MER mission to create color images.
  This video shows one method of how to do it using corel photopaint 8.

The general concept with the Spirit or Opportunity rovers is to combine the separate pancam images they provide to us into a single color image. Each of the images available is generally taken by the cameras using different color filters. The right filters are a bit odd, using some extreme colors, so I won't approach that topic in this more basic guide. However, the left camera filters are more normal. So, you want to look for images whose identification numbers end in: L2L3L4L5L6L7. These are your more normal color filtered images which you will use to process a color image.  This is where to look for the filter numbers:
(More information about decoding these image file numbers can be found at the exploratorium website).

L2, L3, and L4 are more or less your red filters (with L2 being infrared and L4 being relatively close to RGB red).

L5 is very close to RGB green.

L6 and L7 are more or less your blue filters, with L6 being closest to RGB blue, and L7 being closer to ultraviolet.
Take a look at this page for a color approximation chart.

As a simple example, you may only have three of those filters available. For instance, L4, L5, AND L6. These are all you need to process a near RGB color image. The general idea is that the L4 is placed into a red layer, the L5 is placed into a green layer, and the L6 is placed into a blue layer. The layers are then combined into a single full RGB color image.

Using the same method, you can use L2 or L3 as the red layer, or L7 as your blue layer. More complex methods combine all the reds first into a single image; and both blues first into a single image, and then use their yields as the relevant color layer. There are many more complex methods, but these are just the basics to get you started.
Here are the two images shown in the video above:


Gimp is free software that has some useful applications.  The details of how to do this vary from one program to another. But since some people may want to use gimp, and it is free, I've copied and pasted the tutorial I found below.
Here are the direct links to the gimp tutorial:

Making Color Images withThe GIMP
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Las Cumbres Observatory Global Telescope NetworkColor Imaging: The GIMPIntroductionThese instructions will explain how to use The GIMP to take those three imagesand composite them to make a color image. Users will also learn how to dealwith minor imperfections in their images.Note: The GIMP cannot handle FITS files effectively, so to produce a colorimage, users will have needed to process FITS files and saved them asgrayscale TIFF or JPEG files as outlined in the Basic Imaging section.Separately filtered FITS files are available for you to use on the Color Imagingpage.The GIMP can be downloaded for Windows, Mac, and Linux from: http://www.gimp.orgLoading ImagesLoading TIFF/JPEG FilesUsers will be processing three separate images to make the RGB color images.When opening files in The GIMP, you can select multiple files at once by holdingthe Ctrl button and clicking on the TIFF or JPEG files you wish to use to make acolor image. Go to File > Open.Image ModeRGB ModeBecause these images are saved as grayscale, all three images need to beconverted to RGB. This is because color images are made from (R)ed, (G)reen,and (B)lue picture elements (pixels). The different shades of gray in the imageshow the intensity of light in each of the wavelengths through the red, green, andblue filters. The colors themselves are not recorded in the image.Adding Color InformationFor the moment, these images are just grayscale. This is because you areactually looking at the intensity of light in those colors and not the colorsthemselves.Add Color InformationFor each image, go to Filters > Colors > Colorfy; then select the relevant colorfor the image.The common forms of hexadecimal and decimal codes are as follows for eachcolor:Page 2 of 7
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Las Cumbres Observatory Global Telescope NetworkColor Imaging: The GIMPColorDecimalHexadecimalRed255 0 0FF0000Green0 255 000FF00Blue0 0 2550000FFLayering the ImagesNow that the color information has been added to the images, the next step is tocombine the three to create a beautiful color image.This is a simple process of copying and pasting two images on top of the third.Layer the ImagesSelect the green image, go Select > All (or Ctrl+A from the keyboard) then, Edit> Copy (or Ctrl+C from the keyboard)Select the red image, go Edit > Paste (or Ctrl+V from the keyboard), this willpaste the green image on top of the red.Important: In the layers window a new layer called “Floating Selection” or“Pasted Layer” will appear. Double click on the name and rename the layer to“Green”. Not changing the name will cause that first pasted layer to delete whenthe second layer is pasted over it.Tip: If the layers window is not visible, click on any image and go to Dialogues >Layers, or the little arrow next to the close box in the window below the tool box.Next, copy the blue image on top of the green/red image and rename the newlayer to “Blue”.LayersPage 3 of 7
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Las Cumbres Observatory Global Telescope NetworkColor Imaging: The GIMPIn the layers window, you should see that your image now has three layers:“Background” (or “Red” if you changed it), “Green”, and “Blue”.Layer ModeAt the moment you will only be able to see the very top layer, this does not makefor a good color image! But the layer mode can be altered so that all the layersare visible at once.To change the layer modes, select the top layer in the image. Just above thelayers list there is a drop down menu called Mode – thedefault selection for which is Normal. Click on this boxand change the mode from Normal to Screen.This behaves like some video projectors where threeseparate red, green, and blue bulbs project onto onescreen to give a color picture, or an overhead projector(or slide machine) with three transparencies (or slides)(red, green, and blue) stacked on top of each other.Then select the second layer, and change that mode toScreen. You do not need to change the mode of thevery bottom layer.Aligning the LayersIt is unlikely that the alignment of the layers will be perfect; you will probably findthat you can see the separate layers, sticking out from each other (normallyvisible around the stars).The GIMP has a built in Move Toolavailable so thatyou can move the layers easily.First, use the Zoom Tooland zoom in close to astar so that you can see the individual pixels. Thenselect the move tool from the main GIMP menu.Next, identify which layer(s) require moving. Click onthat layer in the layers window. Using your mouse, click and drag the mainimage to move that selected layer. For finer movement you can use thekeyboard arrows. It may make it easier to hide on of the layers while you movethe other in respect to the background. To do this, click the image of an eye nextto each layer name.FlattenPage 4 of 7
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Las Cumbres Observatory Global Telescope NetworkColor Imaging: The GIMPNow that the layers have been aligned, the image must be flattened. Thismerges the three separate layers into a single one. This reduces the file sizewhen it is saved and is more compatible with other applications. You also needto flatten the image before any further processing or the changes that you makewon’t apply to the whole image, just the one layer that is selected.To flatten the image, go to Image > Flatten Image.SavingSave the ImageTo save the final image, go to File > Save As.Navigate to the directory under which you want to save the image (you may haveto click “Browse for other folders”) and type in an appropriate filename. Forexample, “m1_rbg”.Save the file as a TIFF or JPEG from the extendible menu (click on “Select FileType (By Extension)).Tip: If you wish to retain the images so you can come back and edit them later,you can save your image (see saving below) as a Gimp Image (file extension.xcf)Correcting ImperfectionsYour image should now look like a proper color image. However, someimperfections may be visible in the image.Hot PixelsHot pixels are created by individual sensors on the CCD with a higher rate ofcharge leakage (the pixel will think that it is being hit with photons even if it isn’t)than its neighbors. Depending on the sensitivity of the pixels, there is a specificthreshold in which these hot pixels won’t be noticed. Once the exposure timehas gone above the threshold time, the hot pixels will become more prominent inthe final image. The longer the exposure time, the brighter the pixels maybecome. This can also become more of a problem if the pixels are oversensitive.CloningPage 5 of 7
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Las Cumbres Observatory Global Telescope NetworkColor Imaging: The GIMPThe GIMP has a tool that can be used to remove these hot pixels, the CloneTool.First, zoom in so you can see the hot pixel and some surrounding sky. Thenselect the Clone Tool.The idea is to clone the pixels that make up thebackground sky, and copy them over the hot pixel areas –hold the Ctrl key and click on an area of sky.Release the Ctrl key and use the normal left mouse buttonto paint over the hot pixels. You can change the brushsize in the Clone menu window.Color LevelsAdjust Color LevelsDepending on how the images have been processed, some colors of the imagecould be quite poor.The color levels can be adjusted at Tools >Color Tools > Levels.You can change the Channel to adjust. You canchange the image as a whole or levelsindividually from the drop down menu.Alter the input and output levels using the slidingbars on the histogram.SharpeningUnsharp Masking (USM)Images from the Faulkes Telescopes within the Las Cumbres Observatories areof quite a high resolution which may add a lot of detail to an image, but it doesn’tadd sharpness. Sharpness in an image makes edges clear and distinct and thiscan help elicit objects and features in an image.USM works by exaggerating the light-dark contrast between the two sides of anedge by using an algorithm that goes through the whole image, one pixel at atime and deciding whether or not to change that pixel.To apply a USM in The GIMP, go to Filters > Enhance > Unsharp Mask.Page 6 of 7
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Las Cumbres Observatory Global Telescope NetworkColor Imaging: The GIMPApplying a USM effectively is a bit hit-and-miss as there are three separate optionsto alter: Amount, Radius, andThreshold. Users should experimentwith different levels for each option to seewhich gives the best result for their image.Generally, the radius option is the mostimportant so it is recommended you setthat first and change the others after.Sharpening should be applied as the laststep for your image before printing. Thatmeans not sharpening until all otheradjustments have been made to theimage. To achieve optimal results it isalso best to sharpen your image after youhave re-sized it for printing.Congratulations! You have just created a color image!Page 7 of 7
Here's another gimp tutorial online that may be easier to follow than the first one I posted:

For more information on how to do this from a space imaging expert, read this article below by Keith Laney:
Special thanks to "Spendrift" and "Dead Bees" - whose music I found at the free music archive under a creative commons license:
Link to images used in this video:
Image credit:  NASA/JPL
Raw Images Courtesy of NASA and ESA.  Special thanks to EVERYONE who has contributed in the search for Mars anomalies.  Also thanks to Google, Picasa, ImageShack, and Photobucket. 
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Main FeaturesWork in ProgressImage Processing and Miscellaneous ArticlesYouTube VideosResearch Mars at NASA and ESA websitesMars Anomaly WebsitesMars Anomaly Message Boards