I use a band of color coded electrical tape. I buy package of assorted colors and then stick to code. Cromos are pretty impossible to differentiate by looks and same material in rough after heat treatment, rough turned or drawn can look very different. And you will not notice difference with 42CrMo4 or 34CrNiMo6 in bar form, until you mix these with free maching steels and then break drill or tap.
Color coded tape is good enough for me and no trouble allowing for overlaping colors with steel and aluminium, they look pretty different.
Paints are good, when you have a bunch of material and can paint them all on one go, but assortement of unreliable half empty spray cans is not my thing.
Pekka