The first one I can think of is white, which isn’t part of the spectrum. Black is sort of a special case but we all think of it as a color. Are there any others? Is brown in the color spectrum? I guess any color mixed with white (e.g. red+white=pink) isn’t part of the spectrum, which technically means that there are an infinite number of non-spectrum colors.
I am not entirely sure what you mean by “the color spectrum.” ROYGBIV is somewhat arbitrary and certainly not universal.
The most obvious is Purple, which isn’t a spectral color, but is the mixture of blue and red (not to be confused with the spectral colors Indigo and Violet)
the colors you can see can be represented in various ways, but the CIE Chromaticity Diagram is a convenient way. The outside is bounded by the Spectral Locus and the Purple Line joining the ends of the locus. All the spectral colors lie on that line, and all the colors you can see lie within, including White. Technically, in answer to your question, everything on the diagram not on that spectral locus is a color we see that isn’t on the spectrum. That includes light versions of the spectral colors, and blends like purple. Black is the absence of color.
I’ve never been sure about Brown, though. It’s not as if I can point to a place on the CIE diagram that’s obviously brown, although some browns seem to be very much like dark versions of yellow or orange.
All of them. The response bands of the three different types of cone cells in our eyes overlap, so even pure wavelengths are experienced as “mixes”.
Perhaps a more scientific re-phrasing of the question would be: What colors that we see can’t be produced by a single wave-length?
For that question, everything other than the outer edge of the curve in the standard CIE diagram is not representable as a single wavelength of light.
The color spectrum as we know it is defined by the colors a human can see. Colors are an artificial thing. For instance, we have people who are tetrachromats who see more colors than average and others are color blind. Even then the system is artificial. For instance, you may say purple is a color, but its just our red and blue receptors confusing each other.
You can make a list showing the different types of photons at different wavelengths but that wont match up with what you think is color either.
When we were covering the spectrum and the primary and secondary colors in high school physics, I asked my physics teacher, “What about brown, sir?”
He looked dubious and murmured “Well, its not in the spectrum,” and then said “That’s a good question, ask me again next week.”
I asked him again the next week. He said “That’s a stupid question. Shut up.”
I have since learned that brown (and its variants, like tan, or, beige) is a color that can only be seen by way of its contrast with other colors around it (and, of course, in nearly all real life circumstances, there are other colors around). If the entire visual field is filled with something that might otherwise look brown, it will not look brown, but, more likely, some sort of orange or yellow.
Another example of a non-spectral color is pink. I think, like brown, it depends on contrast effects with other surrounding colors to be seen. Of course, virtually all the colors named by less common color words – chartreuse, tope, puce, burgundy, periwinkle, turquoise, etc. etc. – are also not in the spectrum. (Furthermore, indigo really only got on the list of spectral colors because Newton wanted to bring the number up to a nice, mystic, seven. He believed that a beam of white light consisted of a stream of seven distinct types of particle, each responsible for one of the seven colors of the rainbow, rather than a continuous spread of wavelengths as we believe today.)
Although it is true that the response bands of the three different types of cone cells overlap (very extensively, in fact), that has nothing to do with whether colors are experienced as pure. Indeed, color experience has very little to do with the properties of the individual cone cell classes. It arises from the complex way the signals from the different types of cone interact.
Psychophysicists (not the same thing as mad scientists) have done extensive experiments that show that people do consistently experience certain wavelengths or mixtures of wavelengths as subjectively pure colors (such as blue or red), and others as ‘mixed’ colors such as orange or violet. This arises from the visual physiology, however, and has very little to do with the physics of the light. Pure, monochromatic light (i.e., all of the same wavelength) does not necessarily (or even usually) produce an experience of a subjectively pure color, and certain mixtures of wavelengths can produce experiences of pure colors. Indeed, different mixtures of wavelengths can some times produce colors that are subjectively completely indistinguishable (this is known as metamerism), and I am pretty sure I remember being told (by someone who was a real expert on this stuff) that there is no spectrally pure wavelength of light capable of producing a sensation of at least one of the primary colors (I think red, but I am not sure about that). That is to say, no wavelength of light corresponds to true red (if it is red). Any single wavelength of reddish light you show someone will be judged to be a little bit too orange or a little bit too purple looking, and you can only get true red by mixing in some different wavelengths (and there is more than one way to do it).