We’re all familiar with specifying colours in CSS using by defining the mix of red, green and blue light required to achieve our tone. This is fine and dandy, but whatever values we specify have one thing in common – the colours are all solid, flat, and well, a bit boring. CSS3 introduces a couple of new ways to specify colours, and one of those is RGBA. The A stands for Alpha, which refers to the level of opacity of the colour, or to put it another way, the amount of transparency. This means that we can set not only the red, green and blue values, but also control how much of what’s behind the colour shows through. Like with layers in Photoshop…
Direct link: Working With RGBA Colour