What is Color as a perception?

Example 11.3 (Two yellows)

The $589\,\mathrm{nm}$ line of a sodium lamp excites the red and green cones almost equally: the brain says yellow. A screen lighting its red ($610\,\mathrm{nm}$) and green ($540\,\mathrm{nm}$) subpixels gives the same yellow with no light near $589\,\mathrm{nm}$. A prism separates the two instantly; the eye never can.