My guess is this isn't going to be easy. When you add an object to a layer Maya locks the overrideEnabled attr and has the layer manage the Drawing Overrides.
You could try unlocking the attr and changing the color, but I have a feeling Maya isn't going to be to happy about it.
Hmm... just a thought, but you could change the layer color to be "none" and then change the colors on all objects in the layer. (not a great solution)
--g