If you want to convert an existing color value (defined as either a variable or hex value) to an RGBA value (including an alpha layer for transparency), use the rgba function:

background-color: rgba(red,.9);  

This will produce following CSS:

background-color: rgba(255, 0, 0, 0.9);

The two arguments needed are the color and then the value of the alpha layer (0 is entirely transparent, 1 is entirely opaque, .5 is halfway between the two).

From a practical point of view, it's similar to transparentize/fade-out and opacify/fade-in.