I’ve resorted to using HSV instead of RGB to get a Global Brightness function on devices like the Interstate 75. V becomes the brightness variable. It’s a bit of a PITA, but it works OK. You just have to go look up your colors.
Reinvented Color Wheel Picker Demo Page (cssscript.com)
Interstate 74 W, how do you control the display brightness? - Support - Pimoroni Buccaneers