The discoloration on a stainless steel cooking pot is called “rainbowing” which can be caused by exposure to high heat, cooking starchy food (e.g. rice, pasta) or even detergent staining after hand or machine wash. This is common in all stainless steel cookware. It is normal and harmless.
It can be easily removed by using a non-abrasive stainless steel cleaner, which will not only remove the marks it will help to retain the original brightness.
Another option is to cover the bottom with white vinegar. Allow the vinegar to sit for 5 minutes, remove, and rinse the inner pot. The discoloration marks should be removed completely.
We recommend avoiding steel wool, as it will scratch the surface.