In fact, in the '50s and '60s National Geographic photographers often carried with them red scarves or shirts for use when the color didn't happen to be around "naturally." (That practice, today, is frowned upon but likely not strictly enforced... if you know what I mean!)
Every now and then you do get lucky, though. I was making my rounds down at the canal yesterday when in the distance I saw this fellow on a bike headed my way. I quickly got myself into position, composed the shot, then waited for him to enter the scene.

Looking at it now, I think I should have laid flat on the ground. This would have eliminated the light brown path he's riding on. It's a bit of a distraction I think. Ah well, next time!
Cheers.