Hey there! There is only a chance of a mutation if the most recessive slot of the punnett square is picked (25% chance). Then there is a 50% chance of the recessive trait mutating to a more recessive trait. So a [Brown,green] and [brown,green] could get you a [green,green]. Then if you are lucky, that [green,green] could mutate to a [green,blue].
It will only mutate the most recessive of the two traits to a more recessive trait. So if you have [brown,brown] and [brown,green], the most recessive slot would be [brown,green]. Then if that mutated, it would become [brown,blue].