I was just checking out what's new at aircorps library and bumped into this:
It says it was approved in January 1942, with last amendments in August 1949.
There's also a second page with styles and sizes explained.
I don't want to put it here in full resolution, there might be some copyright issues involved.
But here are the colors that may be useful for WWII aircraft:
Fuel system - Red
Water injection system - Red/Grey/Red
Lubrication system - Yellow
Hydraulic system - Blue/Yellow
Instrument Air Vacuum - Orange/Grey
Coolant System - Blue
Breathing oxygen - Green
Air conditioning - Brown/Grey
Fire protection - Brown
De-icing system - Grey
Well, I hope that helps.