Ah, in the foundry we see colour from d-electrons jumping between energy levels when light hits. Copper two plus has incomplete d-orbitals, so it absorbs and gives colour, while zinc two plus has full d-orbitals—no jump, no colour. But from my furnace logs, even pure zinc vapour shows a faint blue glow when overheated, which the textbooks never sit to watch. So I say no ion is truly colourless; we just haven’t pushed it hard enough in our crucibles.
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