+1
Also perhaps another type of color filter could be added. Currently there are "Are exactly", "Contain one or more of", and "Contain nothing other than". Perhaps there could also be a "Contain at least one of each of"? In other words, I might search for black and red, and want to find cards that are both red and black, and I'm open to other colors being in there too.
This on top of the color identity vs. color, as the OP said.