Can two green fish mate and have orange offspring?

Why or why not?

Yes, it is possible.

Explanation:

A punnet square can be described as a diagram which is used to check the outcomes of a cross between parents. According to this cross, it might be that the green fishes that mate are heterozygous for the green trait. When such fishes mate, some of their offsprings might be recessive for the other trait and have that phenotype. Hence, green fishes can mate and produce orange offsprings.