Why are some substances coloured?

There are many reasons why substances appear coloured but for most physical materials it is because the absorption and scattering properties of the material are different for different wavelengths of light. Thus a substance that appears yellow may do so because it absorbs most strongly in the blue part of the spectrum and scatters most strongly in the red and green parts of the spectrum. It is often the case that a pigment scatters light most efficiently in one region of the spectrum whilst having its main absorption band in another. This explains why translucent and transparent coloured films can have different hues when viewed by reflected as opposed to transmitted light.