I have no idea how to create a menulet, but I have been told it is the only way to achieve the desired goal:
A battery icon in the menu bar that is coloured red when low, orange when charging, and green when fully charged.
I tried replacing the files in the original menulet resources folder. (Mac Hd/System/Library/CoreServices/Menu Extras/Battery.menu/Contents/Resources)
It works in the login menu, but nowhere else.