Above: an example of how colours of similar values interact. Due to the surrounding colours, the colour of the top diagonal shape in each rectangle block looks more similar to the most bottom rectangle. In fact, the diagonal shape in each block of rectangles is the same colour at the top and the bottom.
Above: a single colour that looks like two (or in some cases more than two). Each horizontal bar is one colour. Due to the large gradient rectangle that each bar sits on, it seems like the bar is more than just the one colour. As can be seen in this experiment, some colours seem to hold their true colour more against the gradient than others.
Above: a middle mixture. This experiment will work for each of the four colour groups. Stare at a middle panel of a rectangle for at least 20 seconds. What you should see here is that the central colour should start to appear like two colours. The colour at right should creep through on the left edge of the middle colour panel and the colour at left should creep through on the right edge of the middle panel.