Dear CJ1001,
As far as I know there is no standard for painting flanges.
In order to identify different materials we usually have a code. Eg to differentiate between A105 and A182 or A350 etc.
they all appear the same from a distance.
This paint in the form of bands is applied on the rim of the flanges. In any case never on the Gasket Seating surface.
NRP12345
The purpose of ANSI flange standards is to ensure uniform sizing, dimensions fabrication and testing........in otherwords, so that equal flanges will match and give you equal strength.
You can paint them any way you desire.....
There is a standard dealing with uniform color-coding of piping systems. Is this really your question ???
