Prep, prime, and then paint your walls the brightest, most pure white you can get. "In theory," flat finish - but you'll probably be better off with an eggshell finish unless you want to repaint before each grow, because you'll be able to clean that.
BEHR Ultra Pure White (available at Home Depot) is pretty good paint, and it's available in a bright white. Unfortunately, I cannot recall the code number. But tell the person at the paint supplies counter what you want, and he/she will fix you up. EDIT: I think the code number for it is 1,850. Or PP100. I think "#PP100" is, specifically, a small sample container of it, and 1850 is the code for the color, but cannot swear to it right now.
Or go "one step beyond," and buy Orca Film. But it's not exactly cheap. On the other hand, it's probably the most light-reflective product you'll find unless you pursue specialist routes (e.g., high-dollar photography studio / theater stuff) - and it's a lot quicker to install than a gallon of primer and a gallon of paint.
If you want to add a lower color temperature content to your lights, just buy such a light. You don't need 1,000+ watts just to boost a certain portion of the spectrum, after all. Like when a person decides to add UV content to a garden that already has lighting installed... they don't double the wattage, with the addition being UV - they (for best results) max out at probably 5.5% of the total. Or thereabouts; I'd have to dig through my notes to be certain of that figure. But 4% to 5.5% is a good range if you want to run the UV via the same timer that the rest of your lighting is on, IMHO, instead of trying to "blast" your plants with too much UV but for only part of the day (which I wouldn't recommend).
