The screen init (C++ bindings) was trouncing the PWM setup (Python) for the RGB LED.
This is because the backlight pin (12) and LED G pin (13) share the same PWM slice.
This does not seem to affect the screen backlight functionality.
Add a pimoroni.py module which includes Python code equivilents of the RGBLED and Button C++ drivers.
This is simpler than binding these drivers into MicroPython and much easier to maintain/extend.