1. A substance exhibits colour when it absorbs the light of a particular wave length in the visible region and transmit the rest of the visible light.
2. When this transmitted light enters our eye, our brain recognises its colour. The colour of the transmitted light is given by the complementary colour of the absorbed light.
3. For e.g., the hydrate copper (II) ion is blue in colour as it absorbs orange light and transmit its complementary colour, blue.
