FUCK MYLAR
Flat White Latex based paint. Really cheap, one can is around 5 bucks USD, and will last awhile. Have had mine fer 5 years now, painted many a grow box and grow spaces. You can pick it up from your local hardware store fer cheap I'm sure.
Be sure to get latex based paint, not oil based. Also be sure to put 3-4 coats on the area the first round. Apply another coat after every grow/harvest, or every 8-12 months.
Mylar = 98% reflective
Paint = 95% reflictive
Mylar loses its reflectivity over time, at a fast rate. The paint will not go dull for some time. For instance, after a years use of mylar, there's prolly 75%-80% reflectivity left, where the paint might have dropped a digit or two. Oh, and it's sure a hell of alot cheaper, and first-time-grower friendly(Have seen some bad mylar setups.... lol).