Well the only problem with that is that poster board is actually 70% translucent with cloudy day foot candles(2500 lumens/square foot) which is less that the sufficient amount of light to grow, so it would be even brighter. By the way, I'm not an expert just had recent project that involved using white poster board and the amount of light that was transmitted through it.

But in a more positive direction. Try matte or museum board(matte board that is just the white paper the whole way through) Both are completely white, at least on one side(the white matte board sometimes has like a cream color on one side) but one nice thick coat of white primer makes it extremely white. Its quite sturdy as well, about 5.99 for a 36x48 sheet at almost any craft store. Go to the picture frame/canvas area.

-C