Someone mentioned it, it's definitely a bug, apparently un-tuning the car should sort it back out (possibly not if you updated whilst they were white, don't know...). I think it was related to maximum PP or something...
Yes, but I detuned it, reseted to default and nothing happen, my Rosso Scuderia F430 and Dark Blue Metallic Ford GT with white stripes turned into white permanently
Its probably have something to do with the default texture template of the cars ( which is most probably the white theme). Clearly a bug and oversight of the devs. That's why Prologue's out there; so its not in GT5.