I works in some way in fact.
Go to Apperance Panel :
There is Diffuse parameter : Try to pick up a color , if the color don't change in the slot , and values stays at 255, change them manually.
You should have now a color for diffuse, and it will be visible on editor (if not launch a lightmap render))
The "Color" slot is useless, it should be removed as it only brings confusion.