Well if you were going to shell out for just one, I would go with agouti.

IMO Its VERY obvious that hes not chestnut, so the only thing that it would tell you would be if he is heterozygous or homozygous black which doesnt impact his color at all. Testing for agouti will let you know if hes black or bay/brown. (It wont tell you if hes bay or brown, but it will tell you if hes black or not.) You wont know if hes smoky or not.

Ariel: This women, who broke your heart. Do you still lover her?
Hook: Yes
Ariel: Then swear to me on her name.
Hook: I swear on Emma Swan