That section is really about why it is difficult to print black using the subtractive colours C M Y.
It is white light (R + G + B) that falls on paper + pigment.
You have a splodge of 'cyan' ink on a sheet of 'white' paper
Red is absorbed (subtracted), the eye RGB (additive) sees G+B = cyan.
If the same paper is in a room illuminated by red light, then the 'cyan' splodge subtracts any red and the eye sees nothing (black), The eye (RGB) has nothing to add to the red reflected from the paper.
At least that is the way I understand it.
Quote:Green + Blue = Cyan #got it.
Cyan falls on paper+pigment and is absorbed #got it
It is white light (R + G + B) that falls on paper + pigment.
You have a splodge of 'cyan' ink on a sheet of 'white' paper
Red is absorbed (subtracted), the eye RGB (additive) sees G+B = cyan.
If the same paper is in a room illuminated by red light, then the 'cyan' splodge subtracts any red and the eye sees nothing (black), The eye (RGB) has nothing to add to the red reflected from the paper.
At least that is the way I understand it.