Most seahorses that I have direct knowledge of are chameleon-like, able to change color rapidly, from solid to barred, and from light to dark, in a matter of minutes. Substantial changes, from brownish grey to oranges, yellows, and reds take longer, and are generally reflective of their environment, enabling them to better blend in. Put a few seahorses in a bare tank with some white coral skeletons, and they will almost all turn white themselves after a couple of months, some more than others though of the same species.
I'm unaware of any gender color distinctions among the 3 or 4 species I am very familiar with, though there are sometimes subtle shape and size distinctions, along with the obvious male pouch, which can be seen when they are less than half grown. The seahorses I know well are extremely variable in color and appendages, and there are probably gender differences in species I am unfamiliar with.
Local (NJ) seahorses are almost always founds in pods or small schools, and those found as lone individuals, especially in deep water or in areas subject to strong currents were probably swept away from their group by strong currents. I have also found them in the stomachs of large predatory fishes.
I've been a fish collector and a marine aquarist since the 1950s. As time has gone by I find myself less able to tolerate the cavalier attitude many aquarists have regarding their charges. I keep only two very large marine aquaria, populated lightly only by fish I've collected myself, here in NJ, a few in the Caribbean, and two in the South Pacific. I've had these two Pacific clownfish for more than 12 years. To me, a fish should thrive in captivity for at least several years before the aquarist can be considered to have done a good, ethical job. Currently, in 350 gallons of SW in two aquariums I have a total of 11 fish and a few invertebrates.
At one time I bred and raised the local US East Coast seahorse, Hippocampus erectus. This is a difficult, demanding task. I no longer keep any seahorses, and have also stopped giving those I catch to friends, except for one individual who is involved in breeding programs. The rest go free.