Great subject!
I have been told that seahorses change color as a defense, to camoflauge. To me, that translates to stress.
Every seahorse I have had (total of 5 over 15 years) has been black (dark brown) at purchase and has not changed color. Once in a while, do to some ignorant move on my part, they have looked a bit faded, but never a big change, and never permanent.
I would bet my bottom dollar that a well planted seahorse tank is healthier (trate absorbers) and more abundant with live food (pods).
Just my opinion, any thoughts?