The Ospho I use doesn't leave a discoloration or change the rust to iron phosphate. It dissolves the rust and leaves it clean looking. I have used other products that convert the rust but my Ospho dissolves it leaving shiny metal. A Web search shows that Ospho now has many products so you may be describing one of their other products. Mine is a light blue liquid.
You may have to polish after the Ospho dissolves the rust depending if it's a deep rust or a surface rust.
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