Super secret squirrel stuff.
In addition to not giving you un-named paints, the game won't give you some if they have a stripe applied. If you want the paint color, buy the solid, no stripe version. A great example of this is the Ford GT.
Part of me wishes I could get a chip from the carbon fibre selection. It seems silly to have on another car, but it would still be cool to make other cars carbon fibre.