Answer
Allopatric speciation could have seen genetic drift lead to different mating preferences between the two populations or differential natural selection for the camouflage properties of the different male color patterns (though this is less likely than the first option). Sympatric speciation might have seen behavior isolation appear due to mutations changing color preference in two populations of the frogs.
Work Step by Step
There are other possible answers-- the key is to be sure to pick appropriate isolating mechanisms and to clearly explain how each would operate in allopatric or sympatric cases.
