7. Imaginary Colours
Stare at the lighthouse on top of the island in the picture above. Really stare at it and don't look anywhere else. After a little while, the image will appear to flip back to its normal colour... until you move your eyes that is and realise that you must've been briefly put under a witch's spell instead.
You've probably seen this illusion before, but the actual reason why the effect occurs tells us a lot about how we perceive reality. Technically, there's no such thing as colour. The colours that we experience are a result of your brain's interpretation of different wavelengths of light - there's nothing inherently "red" about the colour red, your brain has just decided that's what it looks like. We see colour using cone cells in the backs of our eyes.
There are three different types of these cells that are sensitive to red, blue and green light. When these cones are overstimulated by a single colour, they get "tired" and stop working. This temporarily leaves you blind to that colour and your brain relies on the the other cones to "fill in". The upshot of this is that you see the "opposite" complementary colour for a short time. Red is the opposite of green, yellow is the opposite of blue and so on.
For all intents and purposes, you are really "seeing" those colours in the same way you see any other. Reality is an illusion and colour is all in the mind, after all.