Did you race different track? As I understand it, each track sets it's own unique 256 color palette. Every texture used within that track environment will conform to that single 256 color palette, including trucks used within it. Therefore trucks can change colors from track to track! See here:
http://mtm2.com/~forum/pages/Forum10/HTML/000052.html It is believed that the stock tracks, and tracks made with those specific palettes, reserve a range of colors to make the stock trucks look right, but once you get into custom trucks and tracks which all have their own unique 256 color palttes it lead to chaos.
If I may paste something I've written previously (in relation to track making):
<font color="#FAEBD7">If you've ever tried to create custom textures and discovered the colors look completely different in the game than they did in your paint program then you've run into the track palette limitation.
Each MTM2 track has a single 256 color "track palette", this palette is almost always "trackname.act". Ideally, all textures used in a track would match the track palette, if they do not then the colors in the textures will be shifted (in-game) to the nearest matching color in the track palette. Sometimes this approximation is not bad, although many delicate shades of a color may all be rendered as a solid patch of a single color. Sometimes the color approximation is far from the intended color value and the effect is seen as many 'wrong' colored pixels in the game.
If you've ever seen the track loading screen in MTM2 displayed with very odd colors then you have witnessed a track with a custom color palette, the colors are odd because the track color palette is not compatible with the colors in the loading screen image. Also, if you've ever seen trucks change their color from track to track it is due to their textures not matching the different track palettes.</font>
[This message has been edited by Winterkill (edited 24-12-2001).]