There are three methods:
The Tristimulus Method:
A tristimulus colourimeter shines an internal light source onto the surface of the colour sample.The reflected light passes through three filters – red, green and blue – which determine the intensity of these color components.
The reflected light passes through three filters – red, green and blue – which determine the intensity of these colour components.
The Spectral Method:
A spectrophotometer works in a very similar way – but has not only three, but 31 filters that measure 31 different wavelengths of the reflected light. Thus the spectral method is one of the best methods for measuring colour.
Here, a measuring device (or the eye) is used to compare the colour sample with standard colour samples until all samples are considered identical by the observer. However, this is assessed subjectively by the viewer, which is why the other methods of colour measurement are preferred in practice.