How to define and evaluate the colour of a precious stone

During the estimation of a gemstone colour evaluation is a critical aspect is in defining its quality and therefore its price.
Colour can affect 60% of the total value while the remaining 40% is split between cut and clarity.
Colour evaluation is quite subjective while a standard definition is complex, unlike diamonds which are evaluated by comparison with a set of master stones.
To accurately define a colour we have to analyse its three components.

Colour or hue
Tone
Saturation

We also need to add another element that relates to the distribution and coverage of the gemstone.

THE HUE

The definition of the hue is based on its position on a colour grading chart with maximum accuracy as in the picture.
There are many producers of classification systems and colour grading charts. The most accepted are the GIA GEM SET and the Munsell Colour System that are used in many professional price lists to classify the gemstones.

Wimon Manorotkul pictures

The round gemstone in the centre has uniquely a green hue. The Trillion gemstone on the left can be defined yellow green and the oval gemstone on the right, blue-green.

For a ruby the best and most valued colour is vivid red followed by a slightly purple red and orange-red.
Beyond strong violet-red and orange-red is no longer a ruby but a fancy sapphire (sapphires and rubies belong to the same species, the corundum).

“Often in the East countries, pink or orange sapphires that contain a slight hint of red are sold as rubies. An error of assessment of this kind could be very costly so, if in doubt, we advise you to request the gemmological certificate.” 

SATURATION AND TONE
Saturation can be defined as the strength or richness of a colour. Gemstones of the same species often have the same hue but vary in colour saturation and tone.
The tone indicates the degree of brightness or darkness of a colour ranging from very light to very dark.
An intermediate term indicates a good balance between light and dark in the colour of the gemstone which therefore represents the best classification.
The best values ​​for saturation, indicating how intense or bright the colour is, are the highest and are described with the name of the colour and the adjective strong or vivid.

n this example we have 4 sapphires. Gemstone number 1 has low saturation and low tone. Gemstone number 2 has good saturation and medium tone. Number 3 has a higher saturation of the two but with higher tone. Number 4 is darker and its high tone also reduces the saturation.
Every gemstone, in its best quality, has also the highest saturation. We must not exclude, however, the rarity factor. Even colours with low saturation often end to be appreciated because of their peculiarity.