See http://chandra.harvard.edu/edu/formal/age_snr/3color_ds9.html
Part of the instructions tell how to make a composite image from RGB:
Download and install ImageJ and run the program.
Go to File>Open three times to load casared.fits, casagreen.fits and = casablue.fits.
Go to Image>Color>RGB Merge. =46rom the pull-down menus in the new = window, choose the appropriate files to go with each color as shown = below and click OK.
You now have a three-color composite of Cas A. You can use the following = tools to enhance the image to emphasize different features.
* Image>Adjust>Size
* Image>Adjust/Brightness/Contrast
* Image>Adjust/Brightness/Color Balance
* Process>Smooth
* Process>Sharpen