Multiple colored materials

I had this issue for a long time, I've asked people (via PM) with more experience if they knew how to make it work, but also they didn't know how. (or at least didn't tell me )
I also tried several things to make it work but no results so far.

Is it possible, and if how, to make multiple colored materials? I've added some parts to my mods having a other material (and therefore having an other texture map) but those parts won't change colors with the main body color. If I go texturing my new parts I rather only texture the new parts instead of texturing all the parts of the entire vehicle.

I've added an example, in this case those rear doors are modified van's rear doors, thus having the 'van' material. Copying the van part out of the materials.cs into the mod's materials.cs file doesn't work, neither giving the material another name (I thought it might conflict with original van material...)