Update 1 (fixed colors on the sabre and sabre2)
Update 2 (fixed colors on the Ingot, Virgo and Voodoo)

14 different car colors for each vehicle, except for service vehicles.
Changing the carcols.dat file makes the traffic appear in all these colors and the colors are also pickable in mp lobbies

black, grey, beige, white, dark red, light red, dark orange, light orange, yellow, dark green, light green, dark blue, light blue, magenta

Place the file in /GTAIV/Common/Data/ (Don't forget to backup the original)