Okay, an explanation . . . you asked for it . . .
Look inside your Tron 2.0 directory. (Most likely C:\Program Files\Buena Vista Interactive\Tron 2.0.)
There should be a folder called tcdg. Go inside it. Go inside the game folder. Go inside the WEAPONS folder. Go inside DISC. Finally, go inside SKINS_PV.
See the PV_DISC.DTX file? Rename it to PV_DISC.OLD. Copy and paste one of the other .DTX files, whichever color you're after. So you should now have a "Copy of PV_DISC_WHATEVER.DTX" file. Rename it to PV_DISC.DTX.
Ta-da! You now have that color disc in Single Player.