This is the question may be to James or Max, someone who knows MH chemistry in depth. I have noticed that MH lamps which have pure indium iodide, or some indium-containing mixes often have black color of the frozen halide pool. Or sometimes the color is red. Or in some lamps the blue indium spectrum line is definitely present, but the salts pool is completely transparent. My very wild guess is that sometimes mercury reacts with indium iodide forming free indium and mercury iodide. One of mercury iodides is red, and remaining mercury may form a black amalgam with free indium. But this is just a guess. But why sometimes the salts pool is completely colorless, while indium iodide should be at least yellow? Indium dose too low? Or indium salt prefer to condense on the electrodes of the cold lamp? Another interesting thing is that frozen salts in indium lamp definitely change the color as the lamp cools down.