Here are a couple of commands for prototyping querying rgb value from a polygon vertex.
Hope you find them helpful. These examples are in MEL. I can imagine these would be pretty slow in practice but i think they are helpful for prototyping.
If weve got the texture, u,v coordinate we can use colorAtPoint.
If need to find the u v object of a vertex. I think sometimes could get many uv
objects for a single vertex (i'm guessing depends on edge cuts of map)
Here is if have a uv object and want its (u,v) coordinates.
Owen Burgess (mayastation dot typepad dot com)