I'm actually not sure if this might even be possible right now - if so, I didn't find out.
User-defined presets for button colors (normal/selected/...).
Currently, it seems you have to reset the color for every button, because it defaults to some predefined value. There should be a way to preset the colors - perhaps in the button XML?
Or if there is already, a nod to it somewhere in the help, or interface?
You can copy buttons: hold Ctrl key and move one of buttons or right click then copy/paste.
And there is also possible to set colors in XML. Find your button in subdirectory buttons and edit it with any text editor. Normal color you can find in svg part of button definition, the highlighted and selected in parameters. The interface extension for this is already on TODO list.