Go for it! Ubuntu MATE, its icons and themes are open source, you can modify them and share them to your heart's content. Always check the license to be sure.
If your designs (and any others who get involved) produce really polished unofficial themes, who knows... they might make it into the final cut. The maintainer, @Wimpy will call the shots.
I like the idea of green, orange and silver/gray/(dark?) themes around Ubuntu MATE.
You wouldn't be the first to look into constructing the orange "Ubuntu" theme:
(I too like the orange theme, my desktop has Ubuntu-Mono-Dark (Humanity) icons with some symbolic links to fix inconsistencies - I don't think ever got fixed...)
As an example, I also amended two purple icons to green in the Ambiant-MATE icon package, since like you pointed out, was inconsistent:
(They're going to be included for 16.04 -- note that it's taken 1 ½ months to get to this stage, so be prepared to wait)
The maintainer, @Wimpy will call the shots.