I can get at the file where the evil green Start button is stored. (It's C:\Windows\Resources\Themes\Luna\Luna.msstyles; you need to copy the entire directory and the associated .theme file and operate on the copies, or SFP will kill you.) I can open it with a resource editor. I can extract the METALLIC_STARTBUTTON_BMP resource and copy it into Photoshop. I can select the green bits and desaturate. I can copy it back into the resource editor. I can save.
But when I go to use the new theme, it either gives me an error, or drops me into Classic mode.
(For the record, I've tried Visual Studio, plus three other resource editors. No dice on any.)