Yes, I have S3TC textures (my "Game of the Year" edition came with the textures on the second disk).
The colours in OpenGL are darker, 'muddier', and have less vibrance to them. The actual textures themselves have little to do with the colour of the world. I really don't care about standing next to a wall looking at the creases and ripples. For gameplay I find that D3D looks and plays better (I have altered both the D3D and OpenGL sections in UnrealTournament.ini for best visuals with a playable framerate - most of the D3D graphics options are set to 'True').