And the only reason YOU can do that is because you actually care enough to own Sonic Heroes on the PC. I was following the peeps at SFGHQ as they were toying with the files; it ain't hard, and I'm sure I could do it now (I DO have the demo) if I felt like it.
Of course, a color change is about the extent of what I CAN do, since the characters' textures are, frequently really, really small squares of one single color. Advanced textures with all sorts of freckles, wrinkles, and bump-mapping like you wouldn't believe? Half-Life 2 has it, but Heroes doesn't.