SAO Image Processor DS9 is used by both professional and amateur astronomers to analyze images in FITS format. This image processor can be used for photometry and the construction of color images. FITS files have a "header" that gives the person using the image essential information such as exposure time, filter, telescope, and size of the image.
This is a complete set of instructions for DS9:
http://ds9.si.edu/doc/user/rgb/index.html
In 2011, Niles West presented two research projects based on our work with the Spitzer Space Telescope. The link that follows explains how to use DS9 to create color images.
http://coolwiki.ipac.caltech.edu/index.php/Creating_RGB_images_with_DS9
Use the files in the area below and use the DS9 program to make a color image of M42 :)