It ought to be easy to do. There are any number of ways to get a diff between 2 files, then you just need to tag the diff words with a color. The "widget demo" has a text -search demo which shows how to add color tags. Or you can do a google search for "tkdiff". You might get tcl, though.

Here is a way to have multi-colored tags which I wrote in Tk Multicolor text-search with regex. You should easily be able to modify it so you tag words found through a diff routine.