This is generally on older equipment such as the hardware for car doors and varous bits in side copy machines, but I’ve noticed recently on locksets and other hardware that isn’t generally meant to be seen.
It’s a brass-colored metal that often appears to have a texture stamped into it and has the aforementioned rainbows on it.
What metal is this and what is the rainbow? I’ve got my theories, but you’ll probably only laugh at me.
I know the type of metal you mean. It’s a sheet steel - the exact alloy depends on the application - with either a vacuum-deposited or electroplated cadmium-based coating called yellow iridite. The coating acts as a barrier to corrosion, so the steel won’t rust. The rainbow patterns are optical interference patterns, similar to those that are formed in an oil slick on a wet road, or a soap bubble. They appear in places where the iridite coating approaches 1/4 wavelength of visible light in thickness.