To answer this StackOverflow question I wrote this — a small solution based on a never-rendered element. It fills a 1-pixel canvas with the provided fill-style, and then reads the RGBA values of that pixel. It will work with any CSS color -- name, rgba(), hex, or even something more exotic like a gradient or pattern. Invalid colors are always returned as transparent black. Transparent colors are treated as painted on a newly-cleared canvas.
It's been tested in modern-ish versions of IE, Chrome, Safari, and Firefox. The API is:
color_convert.to_hex(color) # Converts color to a hex-based RGB triple; to_hex('red') returns '#ff0000'
color_convert.to_rgba(color) # Converts color to an rgba() string; to_rgba('red') returns 'rgba(255,0,0,1)'
And you'll probably never want it, but you can also