How does a chameleon change its colour?

The upper layer of Chameleon’s skin is transparent but the layer just underneath it has granular yellow, black and red pigments. These granules can easily move from one point in its body to another. When these cells contract, the concentration of the the granules increases at one point turning the colour of the skin into black. And in turn they get scattered throughout the body , other colours are produced. Therefore, the contraction and expansion of its cells change the colour of its skin