I've been working on a script - which I ended up writing in Python - and it's shaping up to work quite nicely:
It is incomplete, as it doesn't make changes to the PNGs that don't have an SVG - so button borders are still green, for example. As noted in the previous posts. 
we can either re-create (or find) the original SVGs that generated them, or try modifying the colours in the raster image.
Yep, I noticed this when I initially looked at the SVGs. Luckily they seem consistent across the board.. I think each one has been accounted for, and the script uses a function to lighten/darken the hex value. Those values in this program will probably need some further tweaking to accurately match the original shades... they might look a bit too flat.
I created a PPA to contain packages for this program (which I've called ubuntu-mate-themes). I'll look at generating packages for each colour and then I guess we're much closer to having it a reality! 
sudo add-apt-repository ppa:lah7/ubuntu-mate-colours
sudo apt install ubuntu-mate-colours-generator
See the README for usage. I'll be looking at automating/building packages for a few colours, so one could install ubuntu-mate-colours-blue and have an up-to-date "blue" variant of the Ambiant-MATE/Radiant-MATE theme.
GitHub is a good place, you can contribute to this new ubuntu-mate-colours repository by requesting changes (pull request) using Git, or create "gists" containing snippets and text to share with others.