First off, let me just say: Thanks for those ideas, they are all really good.

The goal is a tool that can be used by a few different modules, so I was already intending that there would be a checkbox for using gradients which would be visible if desired. If the checkbox is checked, then each color would intensify from the threshold to the next threshold. That has some odd behavior in practice, so it isn't always the best plan. For example, if you have a Green, Yellow, Red sequence like a stop light, people generally don't have an expectation that the green would intensify before changing to Yellow, so there are different behaviors that need to be possible. I've already done that part for a different project, though, so I left it out of this question. The point is to choose a color, from the set that JA showed, and whether or not a gradient derives from that can be optional.

I like the listbox approach, in general. I think that's a more intuitive approach than what I was doing, but I won't get to look at it today.