Determining if a trait is Mendelian
Congratulations! It looks like you have a Mendelian codominant gene on your hands.
This means the gene will produce the full mutant phenotype whenever it is homozygous, and a third phenotype when it is heterozygous. You should be able to tell the genotype by visual examination alone.
It is wise to verify the inheritance pattern with further test crossing, and having independent breeders run the same tests.