Uh oh. Guess what showed up in the sewing room? Except I'm kind of confused. Who are the parents?

I understood how a pink mouse and a blue mouse could have purple babies. But how does a blue-green mouse and a green-blue mouse have a green baby and a blue baby? Tommy has gone off to do the DNA sequencing to determine who the parents are. DNA sequencing looks a lot like sleeping in front of the wood stove, but what do I know?