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This question already has an answer here:

I figured that it is probably having to do with the wavelengths of visible light that are absorbed and transmitted by the ions, but I'm not entirely sure.

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marked as duplicate by Curt F., Klaus-Dieter Warzecha, bon, Jon Custer, Todd Minehardt Jul 20 '16 at 13:30

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  • $\begingroup$ Your assumption about the absorption of light at different wavelengths is on the right track $\endgroup$ – Technetium Jul 20 '16 at 6:34

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