I am talking about how to, given some code in an arbitrary language and you wanting to change the color of some construct, do that with built-in emacs mechanisms. I think going to a web page, pasting your code, tweaking, and then putting some random lisp back into emacs is a waste of effort when Emacs already has this functionality built in.

It's a nice tip, and it could be nice for just changing one or 2 colors, but for totally doing a theme from scratch, I think I'd prefer a color wheel to messing with color hexes.