The material is not what defines the colour for me. I just try to get different colours on all parts so they're easier to set apart. But I design jigs so most are made from steel or aluminium.

I use colours on the holes for instance. Yellow for thread Blue for toleranced holes Dark red for all others

I also use 2 shades of pink for the machined areas. One for the precise machining other for the less precise (clean up)

This way when I look at a part or assembly I can understand rapidly what's going on and is also easier to detect some errors. For example 2 parts attached by yellow holes is something that pops up faster that checking the tree or measuring the holes.