"Morphology" is approximately equivalent to external anatomy.
Morphological is a the adjective form.
"morph" is often used in the herpetocultural community for any color variant, natural or artficial. It is probably derived from the scientific term Polymorphic, which is applied to any group with different phenotypes, especially color variants.
"Mutant" is any animal with a different phenotype than its parents, but is most often used in decribing abnormal variants.