Unfortunately, the GNOME team decided that user choice was - and I'm quoting here - "less important than [GNOME's] branding". Yeah. As a result, any changes to colors are quite time-consuming and involve editing multiple files by hand.
Even more unfortunately, that's just the EASIER half of the problem. Because the GNOME team has no graphics developers, several pieces of the UI use fixed-color images. So to change the color of a theme consistently across every part of it, you also have to edit up to 60 images. That's a massive waste of time which was completely avoidable, but that's how it is.
So while your request is absolutely reasonable in the abstract, and should be quite trivial in practice, it in fact requires the tiny unpaid MATE team to put in hours of work for even just one new color, let alone "a selection" of them.
I've actually recently done some of the first, "easy" part for a theme based on the classic "Clearlooks" so that the colors can be changed in just a couple of minutes (and, potentially, may someday write a little app to make it even easier). But that's only one theme out of the dozen or so that MATE currently ships with, and doesn't address the fixed-color images of the second part.