In that situation skin color would be an codominant trait, but the problem does not describe any codominant inheritance. Instead, in this problem, the dominant allele (A) is spotted and the recessive allele (a) is no spots. So if a frog inherits a dominant allele it will be spotted, but there is no noticeable difference in the phenotype if the frog has one dominant allele (Aa) or two (AA). This is how most of these problems are set up, but a good reminder to always read the problem carefully in case of non-Mendelian inheritance like codominance.
