Changing Vray material diffuse

Hi.

I have a lot of tiles on a surface and I want to randomly give all of them slightly shades of a color.

I found this RhinoScript that let's me select Objects and assign them a colour, so that works when I change the random function.

However, in the render I would like to add a bump map so what I actually want to achieve is to change the diffuse color or assign different materials to the tiles randomly. Is this possible with Grasshopper?

Cheers,

Eirik