Color poses a problem, since we have to distinguish physical color - that is simply certain wavelength, and perceived color - that is the color seen by a human, after it is processed by the brain (specifically, all the squares in the OP are really combinations of red, green, and blue light emitted by our monitors, whose different combinations of intensity make them look as different shades of orange.)
In case of physical color, scientists and/or engineers would typically adopt a convention that a range from such to such wavelength is a "color" see, e.g., electromagnetic spectrum), which means that this range would contain a continuum of possible colors (unless wavelengths are discretized for some reason).
In case of perceived color the color would be decided by consensus of humans. This is subjective as we can see from different number of basic colors existing in different languages - ranging from 2 in Bassa and Pirana languages to 12 in Italian, Russian and Hebrew (English has 11).
A closely related phenomenon is phonological system - that is how all the possible sounds that a human mouth can produce are split into groups that, for the purposes of a given language, interpreted as a single sound.
Another notorious example of different color perception is illustrated below (borrowed from this page):
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/SfaMP.png)