system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do metals that are heated sometimes get that weird rainbow colour effect?

A few posts of welding were on the front page a while ago and the welds all had a rainbow pattern. (Will find a link when off mobile). I'm guessing this has something to do with the heat that is applied?
gpt
That is oxidation caused by heating.  The degree of heating will determine the color by thickness.  Unless you are an expert reading the colors for heat treatment purposes, it's only decorative as it is only a few thousandths thick.