Here’s an optical illusion that has a real effect on the way artists paint. It’s the colour changing square illusion. Hint: the square doesn’t actually change colour.

What happens is that the background changes and the square appears lighter on a dark background and darker on a light background. Artists need to be aware of this phenomena because we can use it to alter the way tones and colours are perceived. So if I want to make a highlight look brighter I can place it next to a dark shadow.

This example shows how we can use tone (i.e. how light or dark something is) to alter what the eye sees. A similar effect can be used by placing a coloured square on a different coloured background. But that’s a post for another day.