Let me first just state this up front: I'm not a themer. Don't ask me about theming, because I don't know the answer. I'm a module developer. Ask me about module development. I probably still don't know the answer, but at least I won't pretend I don't see you when you ask. (Okay, maybe I still might pretend I don't see you.)
However, there is one thing I do know about theming, and that is there are very few Drupal themes that are flexible for the site builder and/or owner. I, as a non-theming module developer, am frequently looking for themes I can make available to site builders and/or owners who know how to click buttons but don't know how to write CSS, and who want websites that aren't ugly and don't look like every other website that isn't ugly. I have very few options (without spending a lot of money hiring someone to create them for me).
I went away from that search for a couple of years, and when I came back, nothing had changed, so I know this is a still serious problem.
So, here I (a non-themer) am to tell you (a themer) how to create themes that are flexible, so that people will pay you lots of money to create websites that aren't ugly.
While I may blather on about other related topics, this talk is intended to focus on one topic, and that is using Drupal's core Color module to make your themes' color schemes flexible.
(Yes, people will use your fabulous theme and the Color module to create ugly websites, but that's okay, because that's what freedom and open source are all about: creating ugly websites.)