INTRODUCING "NOR GATES"
The NOR Gate is the opposite of an OR Gate. If a either lead A or lead B is in a state of 1, C will be in a state of 0. If A and B is in a state of 1, C will be in a state of 0. Only if both A and B are in a state of 0 is C in a state of 1.

Although the above sounds very confining, it really makes it possible to use a NOR Gate to create any other kind of gate (meaning that the NOR Gate is a universal gate. This means that all of the gates on the image to the left could be replaced by a single gate.Next

To make it possible to have either one color or the other drop the same box, you add an OR Gate.