Tan being a dominant gene has had a kind of funny result in this house. I've had a couple of tan meeces, probably champagne tans, bucks, escape and remain on the lam for days. I now have house mice with tans bellies; it does show the incomplete dominance of tan over agouti. I can't bring myself to kill the wild ones outright, so I try to get them in live traps of various sorts so that I can release them near my compost bins. I have been kidded about having 'free-range fancies'.
We have also seen yellow meeces running wild on rare occasion. The wild or half-wild meeces used to break into my tanks with predictable results. If a half-wild litter occurs I usually keep them until they are big enough to fly away, or if they are tractable, I just keep them. I've tried to breed them on occasion, but they seem to be incompatible. Makes me wonder if my wild mice aren't something other than mus musculus...though they should be still be genetically compatible, shouldn't they? It's a puzzle that will remain unsolved, as I've perfected my housing so there should be no more of this random partying in the mousery.
Now I'm working on mouseproofing the whole thing so that I can total control over the environment.