If you paint below 10 C, the paint will not stick. And if you want to paint it with cans, the result will be ... discutable. This is even more true if it's the first time you do it.
If your bumper is ready for paint, why not try a bodyshop ? tell them that you are not rush to have it. They probably will do it at the same time as another one to share not the color, but the clear. It will cost you not mutch and you will have a profesionally done job !
race car souldn't be restricted to race track !