I create two cylinders and color them red & blue. Then I slightly merge then and do a Union of both. 
To the extent that the two cylinders are not merged in form, I want the two top faces to also be red and blue. So I drew a line across this and indeed it shows two separate faces.
The Color tool states Select Faces, Bodies or Groups to color. But nothing happens for my Face and I get the following error message.
You can’t change the color of an edge, sketch or sketch filling.
So is the Tool instruction message incorrect or is something not working correctly? Or am I misunderstanding something?
Thanks,
Tommy