Is It Possible for a Black and Brown Mice to be Mother/Offspring?

Question

I found a small brown mouse in an apartment and it was brown and about the size of a hamster, now I saw a smaller Blck mouse and Im woundering if its the brown ones baby.

Answer 1

It is quite likely they are related, if only because it's unlikely you have two different species of mouse in your apartment.

The usual house pest mouse is the house mouse, mus musculus.  The pet store mouse is the same species, but bred over many generations to be tame and to come in different colors.  I have seen an escaped pet mouse consorting with wild house mice in a friend's apartment.  

However, you say the brown mouse was the size of a hamster.  That's pretty big for a house mouse.  I think it may be a young rat.  If that is true, the smaller mouse was no doubt a younger rat.  You won't find rats and mice in the same house, because the rats drive the mice out, or kill them.