Colour is a construct of the mind. Out in the real world what we perceive as colours is simply a small subset (less than an octave) of the wavelengths in the electromagnetic spectrum. Evolution has endowed us with a mechanism for distinguishing between these wavelengths, thus providing additional useful information about the world around us.
I knew this already. What I didn’t know was that the three pigments in the colour-sensing cells in our retinas are consumed in the process of detecting the colours, and are continually replenished. I found this by watching some very informative videos on the Oscars website. Yes, as in movie Academy Awards. In particular, is has a number of pages dealing with its ‘Solid State Lighting Project‘.
On the right of this page is a section called ‘Symposium Videos’. These are each about five minutes. I recommend ‘Perception of Color’. This (towards the end) makes clear why this optical illusion works (look at the black + in the middle and wait):