how do the colours of a chameleon change when it sits on the objects of different colours?

Most chameleons change between green, brown and gray (their surroundings are usually the same colors). Light, temperature, and emotional state commonly bring about a chameleon's change in color.

Chameleons can change their color because of a very complex cell system. Beneath it's transparent skin are several cell layers. These layers are the source of the chameleon's color change. Some of the layers contain pigments, while others just reflect light to create new colors. By opening and closing cells called melanophores, chameleons change their skin color. The cells direct sunlight to specific pigments which reflects the light back in different colors.