Currently a few widgets do not support colorization. EV_BUTTON is among them. This is because the OS theme engine takes precedence over the internal settings. To fix this, we would have to rewrite the drawing code of buttons and this will be quite a bit of work for a small benefit. Fixing this is therefore a low priority at the moment. 

If there is a real need for color, you can easily build your own buttons using an EV_DRAWING_AREA. The downside is that they won't be standard.