You shoulda heard that HSL is the jam by now.
What I'm trying to say is, all that time that you've fell in love with rgba(), you're gonna do it all over again with hsla(). Harder.
For more on HSL, the theory, check out Coding colors easily using HSL notation by Zoltan.
This little app should help to familiarize you with HSL colors so you can create them on the fly more confidently.
This code is WTFPL or public domain or whatever. IDGAF.