i said that cause i have the same problem...

i think that its not the color reduction... my pcx files are pretty similar to the original image, at least it could be the way the engine renders the images, but i noticed that the volition textures appears instead "normal" in game.
In modview (not sure in pcs) there is a feature called "lights" with some coordinates
I don't know what they are for... volition models have a lot of lights, mine none... maybe this is the cause but i never had time to make some tests so i'm not sure
another possibility may be the graphic card and its settings
(but i didn't test this too)