What Colour is it?
This is another colour illusion. Your eye sees colour in context. The same colours might appear very different if you just change the surroundings. Look at the rings in the video below. The colours are in fact the same in both rings but they look different. All that has changed is the order that the colours sequence. The actual colour is the same. When the cyan is next to the grey, you see a different colour from when the cyan is next to the magenta.