Blaze_Amita, I believe you can breed harlis to brokens to get tri colors, correct? They're useful in breeding, but can't be shown (like Vienna marks in other breeds are useful to bread BEWs, but cannot be shown).
Misty, there are plenty of colors that can be bread together, but also some that cannot. I would look around online - I'm sure someone has written a genetics guide on this (I've seen them for hollands, so I'm sure they're out there for mini rex too).