There a small difference when u use rgba(255,255,255,a),background color becomes more and more lighter as the value of 'a' increase from 0.0 to 1.0. Where as when use rgba(0,0,0,a), the background color becomes more and more darker as the value of 'a' increases from 0.0 to 1.0. Having said that, its clear that both (255,255,255,0) and (0,0,0,0) make background transparent. (255,255,255,1) would make the background completely white where as (0,0,0,1) would make background completely black.