In Black★Rock Shooter, the Otherselves bleed their color theme.Usually hand-waved by having different chemicals in their blood. There are other pigments like hemocyanin or chlorocurorin that come in such vibrant shades as pink, orange, blue, and green, but these are much less efficient at transporting oxygen than hemoglobin and thus only appear in invertebrates, which have smaller bodies and a better surface-area-to-volume ratio, and thus less need of efficient oxygen transport.
This is because hemoglobin contains iron, which is vital for transporting life-sustaining oxygen where it needs to go. Red blood cells are red because they contain a protein chemical called hemoglobin, which is bright red in color. Of course, here on Earth, all vertebrates (with the exception of certain lizards and genetic conditions ), the largest and most complex members of the animal kingdom, have red blood-the reason? Because all animals breathe oxygen. Darker-skinned aliens, or ones with scales, feathers, hair, or something else hiding their skins avert this problem, albeit probably unintentionally.
With Human Aliens, this is a very mild case of Artistic License Biology: Since the skin is translucent, a creature with different-colored blood wouldn't have the same skin color as a human.if the skin is white.