This watercolor painting is of a male Betta, Betta splendens (or Siamese Fighting Fish.) Bettas are anabantids (Derived from the Greek Verb anabaino, meaning "to go up") which physiologically means they have two methods of breathing. Bettas can breathe from their labyrinth organ which enables the fish to breathe from the surface, or they can also breathe through their gills.
This particular male Betta is a Veil tail: extended finnage length and a non-symmetrical, drooping tail; with caudal fin rays that usually only split once. This beautiful tropical fish displays shades of cobalt, turquoise, violet, cerulean and touches of deep red. In a blue Veil tail, the top layer of skin cells is more concentrated with iridocytes, or iridescent pigment cells, so these cells appear as a shimmering blue.
Male Bettas are quite territorial, and will "flare" up their fins and gills in a magnificent display when threatened or irritated. "Flaring" is also how male Bettas impress female Bettas to best show their colors, size and fins.