Hi,
1) Yes the color (actually 2 colors because it is a gradient) is an option when setting up each button mash event. So you can have different colors for different button mash events.
2) You cannot use a picture as the background with the plugin, but you could set the window to be transparent type and show a picture where the button mash window would be. It might be a bit difficult to line up, I can look to add a picture as an option in a future update. You can also change the window's windowskin, back opacity, tone, and padding but those settings currently affect all button mash events.
Also while testing this I noticed that I forgot to hook the background type parameter up 😅you can add this code into the js file to get the background type parameter to work:
CGMZ_Window_ButtonMashProgress.prototype.initialize = function(rect) {
Window_Base.prototype.initialize.call(this, rect);
this._gaugeRect = new Rectangle(0, this.lineHeight(), this.contents.width, this.lineHeight());
this.setBackgroundType(CGMZ.ButtonMash.BackgroundType);
this.clearData();
this.hide();
};