[refurb]

It’s a very fine layer of surface oxidation. Same reason you can get different colors on aluminum depending on how thick the oxide layer is.

Stainless steel is corrosion resistant, not corrosion proof. Dishwashers are actually pretty harsh with high temperatures and high pH detergents.

You can get the same effect by putting a stainless steel spoon in a pot of boiling water.

[sumtechguy]

On top of that some detergents will leave a bit behind (for various reasons). All of the washers I had, have an extra 'jet dry' compartment. It adds a drop in for the last rinse cycle. Seems to bind to the stuff and clear it up by letting it wash away. So if you are getting spotty glasses after cleaning look into that.