For example the bright royal blue color; R0 G10 B220, HEX: #000ae1. I know this color cannot be printed nor painted. But can this blue color exist in the real world other than on a digital screen? For example the aurora in the sky, or a rainbow in the sky, or a blue glass bottle?

  • $\begingroup$ Remember that our vision (and that of RGB screens) are limited to the visual spectrum, which is just a very small part of the electromagnetic spectrum. Since photons with any frequency in the spectrum can be produced, you could have colors that you can't even see with your eyes, and not because of that they stop being real frequencies of light. $\endgroup$ – Charlie Mar 21 '19 at 1:16
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    $\begingroup$ The question may be a nonsequitur. Humans only see 3 colors, based on the cone cells in their retinas. Whether a color can be printed or painted does not change this fact. In addition, the 3 colors that we can see do not constrain in any way the colors that can exist in nature. $\endgroup$ – David White Mar 21 '19 at 2:19

The colours we experience come from photons hitting our eyes and triggering the right amount of activity in different kinds of cone cells. So any source of photons that triggered the same pattern as #000ae1 on your screen triggers would be the same colour. There are presumably many physical processes beside computer screens that could do that, but finding "ready made" examples might be tricky.

We typically only have three kinds of cones mediating our color perception (roughly, we integrate spectral radiance with three different sensitivity functions to get the response $r_{red}=\int L(\lambda) x(\lambda) d\lambda$, $r_{green}=\int L(\lambda) y(\lambda) d\lambda$, $r_{blue}=\int L(\lambda) z(\lambda) d\lambda$ somewhat like in the CIE system). Since this compresses a high-dimensional spectral space into a few responses, there are many spectral radiances that give the same visual response.

However, most objects we see in everyday life reflect or scatter light rather than emit light. This tends to make very pure colours rare, and contribute to pigments working differently from computer screens (pigments work by reducing the re-emission of photons of certain wavelengths giving us subtractive colouring, while screens and stage lights work by adding photons of certain wavelengths giving us additive colour). Since reflected light also picks up colour from the surroundings and our brains do a lot of nontrivial guesswork about the "real" colour this can really make it hard to imitate a particular colour stimuli.

The easiest way of getting #000ae1 is likely some emission or filtering process. An aurora produces a spectrum dependent on particular emission lines of nitrogen and oxygen, so unless we are lucky and they have some combination that exactly fits, auroras are unlikely to do it. Rainbows give a spectrum of sunlight overlaid on the background light: they tend to not be that strongly saturated, likely not getting close to royal blue unless there was a really black background. A blue glass bottle with the right colour might well filter sunlight into royal blue.

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