Why do Objects Have a Certain Colour

The colour of an object should not be a mystery. Is a result of two factors:
1. The size of the molecules and crystals of which the surface of the material is made. Some wavelengths are absorbed by the material and some are reflected - or alternatively destructive interference results for some wavelengths of light, resulting in absorption by the surface, and constructive interference results for other wavelengths, resulting in reflection by the surface.
A material can appear to change colour as it is viewed from different angles. A petrol film appears to change colour at different viewing angles as a result of constructive interference occurring at certain viewing angles between light from the top and bottom of the layer of petrol.
2. The wavelength of light absorbed and re emitted by the electrons in the atoms of which the material is made. Electrons absorb and re emit light of certain wavelengths only, characteristic of each element or molecule. A red object absorbs preferentially all wavelengths of light except red, which it reflects, so we see the object as red.
Materials can often be made to change colour on demand. This happens in liquid crystal displays as a result of a voltage changing the orientation of a crystal.