Why the black grease paint on athletes' faces?

I’ve now got this mental image of an NFL linebacker insisting on playing in his lucky sports bra.

But what I’d really like to see is full-out war paint. Not just a couple of black strips; let’s see some football players with a full-out multi-colored ritual war pattern on his face.

I just wasted 15 minutes looking for a picture of Andrew Bryniarski from The Program. This is the guy. Warning - That leads to a “What movies have naked dudes in them” website, but the link is just a tiny picture of the dude’s head, if you don’t know who the hell I’m talking about.

I want my 15 minutes back.