Is there a universal colour code for screw bits?
The colour codes are NOT universal. There is no agreed upon universal colour code for colouring bits (to my knowledge). 

I have found that each brand (mostly) follows their own colour coding. Powerfix uses the colours you describe, and some brands, either intentionally or otherwise, use the same colours as other brands. 

But mostly, they do not. For example, Halfords uses


Blue: Philips
Yellow: Pozidriv
Red: Flat
Green: Torx


While Wera uses


Red: Phillips
Black: Pozidriv
Green: Torx
Blue: hex
Yellow: flat


And then if all the tools in a set are the same, e.g. hex keys, Wera uses colours for the sizes instead.