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Colours correspond to infinite-dimensional vectors, since there are infinitely many wavelengths of light. But humans can only perceive a three-dimensional projection of colour (red, green, & blue). What's interesting is that it's *not* an orthogonal projection. Here's a plot of the basis vectors.

The 3 color basis vectors are the 3 types of cone cells in the retina The fact that they aren't orthogonal (& you can't have negative light) leads to pop science articles like this. I.e. stimulating individual cone cells lets you perceive colors that don't correspond to any mixture of wavelengths.

There are easier ways to perceive impossible colors than shooting lasers at individual cells. You can cross your eyes so your brain mixes the projections. You can also saturate one type of cell to tire them out.

There are also rod cells in your retina. In principle these give you a 4th dimension for perceiving colour. But they are for peripheral & night vision, so we don't perceive a 4th color dimension. 🤷

The reason I got interested in this is that I didn't understand why red plus blue makes violet/purple. Violet corresponds to the shortest visible wavelengths -- shorter than both blue and red. The explanation is that the S/red cone cells are also activated by short wavelengths for some reason.