Blue, purple, and gold reflections that appear on a stainless steel pot after heating are not dirt or damage. Heat thickens the chromium oxide layer that protects the steel. Once it becomes just the right thickness, it separates incoming light into colors; what you see is thin-film interference, not a painted surface. It does not transfer to food or affect the pot’s use.
What makes stainless steel stainless is not the steel itself but the chromium in it. As soon as chromium meets air, it forms a transparent, extremely thin layer of chromium oxide on the surface. This layer separates the metal from moisture and acid, and it reforms when scratched. It is too thin to see, so it normally remains transparent and does not change the steel’s color. The pot’s resistance to corrosion comes from this invisible layer. In other words, an oxide layer is always present on the surface; the only question is how thick it is.
Heat makes this layer thicker. When an empty pot or one with very little liquid is heated, the oxide layer grows and starts interacting with light once it reaches a certain thickness. Light reflected from the surface mixes with light reflected from beneath the layer; some wavelengths cancel each other out, while others become stronger. That is the color the eye sees. The same effect appears in the thin film of oil on a puddle and in soap bubbles; it is called thin-film interference. The color comes from the layer’s thickness, not paint.
The color sequence is not random; it indicates how much the layer has thickened. First, a pale golden yellow appears. As the heat increases, the shade shifts to brown, then purple, and then blue. That is why colored rings appear only where the flame or heating element touches the pot; the edges and lid stay clean. For the same reason, the discoloration usually forms on the outside, in the band just above the bottom. If it appears inside the pot, that means the pot was left empty or with very little liquid over high heat.
It is important to know that this discoloration is harmless, because attempts to remove it can cause damage. Steel wool, abrasive powder, and hard scrubbing remove the layer; the area stays unprotected until it oxidizes again, and pitting can occur in the meantime. The right method is acid, not abrasion: pour a little vinegar or lemon juice into the pot, wait a few minutes, wipe with a soft cloth, and rinse thoroughly. For stubborn discoloration on the outside, repeat the same process with hot water and dry the pot.
Prevention follows from the same mechanism: since heat causes the color, avoid unnecessary heat. Do not heat the pot empty for a long time, do not choose a burner wider than the pot’s base, and do not leave the pot on the heat after the liquid has evaporated. Stainless steel pots washed in a dishwasher can sometimes develop a different, pale, dull appearance; that is a separate mark left by limescale in the water, and it can also be removed with acid. There is no reason to discard a pot just because you see a colored stain.