Here is something that some of you may have all ready done. And if you haven't, then its really cool.

After you kill any Covenant, their corpse usually falls onto the ground. Then you can walk up to the corpse and press B (Melee Attack) with any of your weapons. Then their bloods and some pieces of flesh will squash out of their dead body.

You get different colors for different Covenant. For example Hunters have orange blood, while Grunts have blue blood. I just did this one day when I was really bored on Halo.

NOTE: THIS IS VERY GORY