[Kong]

Does anyone have any ideas about an easy way to make flash paper that burns different colors??

[jdurg]

I'm not sure how to make flash paper, but you can different colors by soaking the paper with different salts.  Potassium salts burn with a violet color, Strontium and lithium with a red color, sodium with a yellow-orange color, barium with a green color, copper with a blue-green color, and those are all I can think of at the moment.

[Kong]

Flash paper is nitrated cellulose.    

[hmx9123]

I haven't heard of any ways to color flash paper; flash powder (magicians') is a different story, though.  The problem is since it's nitrated cellulose, it doesn't hold salts well.  In addition, if you coated it with solutions of salts and let them dry to try and get a color, unless it was a VERY strong colorant and you had a lot of it, it would be lost in the sodium line when the nitrocellulose burned (with its horribly contaminated sodium salts).  The best place to look is on the archives of rec.pyrotechnics under Google Groups.  You could ask there, but I think they get this question pretty frequently, as well as the "how do I make colored fire to eat" question.