I've always used regular old red. if slightly dark.
blood in a different color just don't look gory and intimidating to me.
men, elves, dwarves and lizardmen as far as i know, all bleed red.
so why not orcs?
blood for the blood god! in all the colors of the rainbow
EDIT: lovely troops, i got a bit carried away with the blood
especially the banner, champion and swordsman (always loved that model, even though the blood looks like it has heavy fuel oil consistency)
the shading on the white and the subtle red highlights are great