I think a white sandbed has little to do with lighting. Although it has an indirect effect on the sandbed. Keeping the white sand look is about having habitants that sift through your sandbed, keeping it clean and continually overturning it. Starfish, hermits, snails (just moving over it), fish, etc all will help maintain this look. All that lighting has to do with is to encourage algae growth which after a tank becomes established isn't a problem.
For the first 3 or 4 months of my tank my sandbed was never clean. Always had something on it, either hair algae, cyno, or something. Now after i've gotten everything in order my inverts and fish keep it clean and it is white 90% of the time. Now the downside to this is you won't have a flat sandbed, i have holes, mounds, and crevises everywhere that my fish and inverts have burrowed.