Just found this great post at the chaosgroup board concerning random colors per object.
Normally i used the vrayUserColor and user attributes to create random hue shifts (more infos here) but this solution sounds even better. need to check it out for myself. as far as i know its only working with the nighly build after the 20th january for now.
thats what yolov wrote: "Here's a quick example of how it works, along with a scene that I'm attaching: - add extra vray attributes to a samplerInfo node (this will expose additional outputs to the samplerInfo) - connect the samplerInfo.vrayRndomByID to a ramp's vCoord (if you're using a vRamp) OR uCoord (for a uRamp) | or both, if you intend to switch between ramp modes --> the ramp's position and color values will be the range of colors to be distributed to the objects --> plug the ramp into a shader and use the same shader on lots of objects"
for more, have a look at the thread.