The last parameter to the rgba() function is the "alpha" or "opacity" parameter. If you set it to 0 it will mean "completely transparent", and the first three parameters (the red, green, and blue channels) won't matter because you won't be able to see the color anyway.
With that in mind, I would choose rgba(0, 0, 0, 0) because:
You could avoid the rgba model altogether and use the transparent keyword instead, which according to w3.org, is equivalent to "transparent black" and should compute to rgba(0, 0, 0, 0). For example:
h1 {
background-color: transparent;
}
This saves you yet another couple bytes while your intentions of using transparency are obvious (in case one is unfamiliar with RGBA).
As of CSS3, you can use the transparent keyword for any CSS property that accepts a color.
I would recommend using rgba(255,255,255,0) because broken (newest) safari thinks that if you are using transparent or rgba(0,0,0,0) in linear-gradent you really mean gray, For more info please head to - What happens in Safari with the transparent color?
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.
There are two ways of storing a color with alpha. The first is exactly as you see it, with each component as-is. The second is to use pre-multiplied alpha, where the color values are multiplied by the alpha after converting it to the range 0.0-1.0; this is done to make compositing easier. Ordinarily you shouldn't notice or care which way is implemented by any particular engine, but there are corner cases where you might, for example if you tried to increase the opacity of the color. If you use rgba(0, 0, 0, 0) you are less likely to to see a difference between the two approaches.
Source: Stackoverflow.com