Why did my gumbo stored in a metal pot refrigerated overnight change?
I can think of a couple answers here:


The larger volume of soup in the pot stayed hot much longer than in tupperware, and so it continued to cook, or fermented/spoiled overnight.
If the pot was aluminum and the gumbo acidic, the two reacted.  I've seen this happen with long-simmered, acidic stocks.  The color changes, and the flavor sometimes does too.