Why do iodide solutions sometimes turn brown when exposed to air?

Although iodide ions are colorless in solution, iodide solutions can develop a brownish tint when exposed to air because the iodide is oxidized to free iodine by atmospheric oxygen.1

This process occurs because the iodide ion acts as a strong reducing agent, meaning it readily gives up an electron. Once free iodine ($I_2$) is formed, it can combine with remaining iodide ions ($I^{-}$) to produce polyiodide ions, such as triiodide ($I_3^{-}$), which accounts for the characteristic brown color observed in the solution.1