Discolored blood

Also any other creatures whom you know to have certain color blood but they have something else instead.

This is actually realistic in some cases, such as cockroaches do bleed yellow, some insects bleed green, and there are real animals that bleed even blue. However, most people probably don't know all that and it will seem strange, so they all are essentially applicable for this despite being the real thing (though can be omitted).

Amusingly this can be used to lower a game's rating. A game with red blood for example gets Teen rating from ESRB while green blood can get Everyone rating.

See also: bloodless, alien blood (when this happens with otherwordly things, monsters, and such)