What are we looking at when we say magenta? Quantitative measurements of RGB and CMYK colours with a homemade spectrophotometer

We address some issues in colour theory, which are of relevance from an educational perspective. Spectra of emitted RGB and of transmitted CMYK colours are quantitatively processed and analysed with quite inexpensive homemade instruments, making use of smartphones as affordable digital cameras. LCD monitors and paper sheets with pigments coming from a laser printer are used to point out the basic differences between additive and subtractive colour formation. As an especially relevant aspect, we point out how it is possible to construct a simple model to explain the subtractive mixing process in terms of convolution of primary colour filters. The analysis presented in this work is particularly suited for enhancing the need for a proper understanding of the physiology of human eye-brain action in light acquisition and perception of colours.