Bluish or “rainbow” discoloration is common on stainless steel cookware including Instant Pot inner pot. This is caused by minerals or salts in the food and water. This discoloration is easily removed by using a non-abrasive stainless steel cleaner, which will help to retain the original brightness and assist with the removal of stubborn marks.
However, the easiest, quickest, and cheapest way to remove the rainbow stain from the inner pot is to cover the bottom of the pot with white vinegar. Let it sit 5 min, pour out, and rinse the pot. The discoloration marks can normally be removed completely. White hard water stains can be removed with a sponge dampened in vinegar or lemon.
The following pictures show our experiment with an Instant Pot inner pot under regular use for about 8 months. We used Heinz’s Pure White Vinegar.
Please avoid using steel wool in cleaning, which will scratch the surface.