I don't know that I agree on dark coloring being an indication of stress, solely. I kept two seahorses for 3 years and never did see a change in color. Surely, they were not stressed for 3 years. They were black/brown at purchase and never varied much.
I have seen red, orange, yellow seahorses in LFS and have not seen them change from those colors after eyeing them for a week or two, nor did the LFS admit to color changes.
I have seen seahorses kept by folks on this message board that did change color and noted that the tanks they were kept in had extremely vibrant colored decorations. In the wild, they reportedly change to match their environ. Much as a chameloen.
Finally, and conversely, I purchased a little seahorse at the LFS yesterday. She was advertised as a yellow, however at the LFS and here, she is as dark as you please. She is not showing any signs of stress, whatsoever.
I have wondered if the lack of threat or stress keeps seahorses from running the color changes. I prefer natural tank decor, but it might be fun to throw in a peice or two of petroleum plants to see if they affect any change.