The simultaneous contrast tool shows the phenomenon of colour contrast enhancement, in which the perception of a colour is changed by a surrounding colour in the direction of its complementary colour. In addition, the saturation and brightness can also be influenced, which also contribute to the contrast enhancement.
For this purpose, the computer screen is divided into two different areas whose colours can be changed interactively. Within each of the two colour areas there is a square of the same colour, which can also be changed independently.
Depending on the surrounding colour, a different colour impression is created, although the squares are physically identical. The colour of the squares changes in the direction of the complementary colour.
To verify that the squares are actually the same colour, they can either be moved to the left or right colour field or the background colour can be hidden.