Colour is the first property geologists consider when identifying a mineral although using this method of identification alone has it's limitations. Brightly coloured minerals tend to be more valuable.

Colour is dependent on light.

Visible light occurs in the form of seven colours (the colours of the rainbow).

When light passes through a mineral it may be absorbed, reflected or pass through. If certain colours of the spectrum are absorbed then it may appear to be coloured.

Therefore, minerals that appear red do so because they have absorbed all the other colours of the spectrum e.g. copper appears green/blue because it absorbs all the other colours.

However this is not the entire story because there are other scientific processes that affect colour.

But minerals appear coloured because they absorb some part of the light.

If all light is absorbed the mineral appears black.

Colour depends on chemical composition but it can be changed by the inclusion of impurities. Therefore some minerals with the same chemical composition may not appear the same colour. This is why relying on colour as the only method of identification is unreliable.