Here is my scenario:
I have 10 items with an uncolored materials, they are UV mapped and have diffuse/normal maps.
I also have 10 or so colored materials, Gold, Silver, Iron…etc.
Is there any way that I can overlay the colored materials on the item’s default materials or will I have to make a separate material for each possible combination?
It depends a little on the exact effect you want to achieve.
One way would be to use the standard shader in Fade mode. Apply all the materials you want to the mesh renderer for the item. For example one uncolored and one Gold material. The Material at the top of the list will be drawn first, then the next material and so on. In this way you would get one texture at the bottom and then other textures overlayed on top.
You can additionally control how much of each of those materials should be visible by adjusting the Alpha color on the Albedo in the Material.
If you’re not afraid to dig into cg a bit you can customize the shader to accept the properties and textures of both materials and render your metal colors instead of your diffuse map according to whatever criteria you want (I assume a second uv map?). I think this particular application would actually be fairly straightforward so it’s not quite as intimidating as it sounds.