I think your pot is discolored from the water. I had the same thing happen to my pot and the next time I simmered fruit to get juice for jelly the discoloration got less and less. Try boiling some vinegar and water in it and then scrub it with an sos pad

I have a set of stainless pots that are about 35 years old. They get pretty hard use. The smallest one has that "rainbow" effect inside but the pot works fine and the outside looks perfect and shiny. so I continue using it. The object is to cook and heat food, not have a perfect looking pot,right?

"Cameo Aluminum and Stainless Steel Cleaner" works great on ss pots and pans.

Look for it at your grocery store. Should be in the same aisle as dishwashing liquid.

It's in a little plastic can, like kitchen cleanser.

Here is a link that might be useful: Cameo Cleaner (scroll down a bit)

Boil a pot of water with at least a cup of vinegar in it. I cleaned the minerals out of my tea kettle this way, and it was like new when done. You will want to open the window for lessening the vinegar smell.