Magenta is colour we see, but one that does not exist in a physical form, it reveals something internal. Our relationship with the 'real' world is through our senses and an internal interpretation. Our eyes are sensitive to wavelengths of light at particular wavelengths, each wavelength of the visible spectrum can be called a colour, using a prism reveals all of these available colours, but magenta is missing.
Isaac Newton noticed that combining the light from 2 prisms and overlapping the blue and red ends of each spectrum creates magenta, this artwork recreates Newton's experiment. Magenta is an extra-spectral colour, meaning it is missing from the spectrum because there is no wavelength for magenta, but our eyes see the combination of two wavelengths and magenta is created by our brain interpreting the two colours in this way.
This simple act demonstrates that our interpretation of the world differs in incredible ways from 'reality', we view the world through a narrow band of senses and even then create an internal image that differs again from this given information.