Diagnosis
Those rainbow or bluish stains are called "heat tint" or oxidation. They're harmless and very common on stainless steel when it's exposed to high heat, minerals in water, or certain foods (like eggs, starches, or acidic ingredients).
How to Fix It
You can remove them easily. Make a paste with baking soda and water or a sprinkle of Bar Keepers Friend powder. Gently scrub the stained area with the paste and a non-abrasive sponge or cloth. Rinse thoroughly. It should come right off.
If That Didn't Work
For tougher stains, fill the pot with a 50/50 mix of water and white vinegar. Bring it to a simmer (use the Sauté function on low) for a few minutes, then let it soak until cool. The acid will dissolve the mineral deposits causing the stain. Scrub and rinse.
Heads Up
Avoid using bleach or abrasive steel wool pads, as they can damage the polished surface of the pot. The stains are purely cosmetic and don't affect cooking or safety.