Why are some substances coloured?

Why? There are many reasons why substances appear coloured but for most physical materials it is because the absorption and/or scattering properties of the material are different for different wavelengths of light. So 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

Why is copper (II) sulphate solution blue? If white light (ordinary sunlight, for example) passes through copper (II) sulphate solution, some wavelengths in the light are absorbed by the solution. Copper (II) ions in solution absorb light in the red region of the spectrum. The light which passes through the solution and out the other side will have all the colours in it except for the red. We see this mixture of wavelengths as pale blue (cyan). Copper (II) sulphate solution is pale blue (cyan) because it absorbs light in the red region of the spectrum. Cyan is the complementary colour of red.

Which substances? Transition metal ions A metal which forms one or more stable ions which have incompletely filled d orbitals Zinc with the electronic structure [Ar] 3d 10 4s 2 doesn't count as a transition metal whichever definition you use. In the metal, it has a full 3d level. When it forms an ion, the 4s electrons are lost - again leaving a completely full 3d level. This shortened version of the Periodic Table shows the first row of the d block, where the 3d orbitals are being filled.

The Origin Of Colour Complex ions containing transition metals are usually coloured, whereas the similar ions from non-transition metals aren't. That suggests that the partly filled d orbitals must be involved in generating the colour in some way.