The "Colour" in the 4Cs diamond grading system is an indicator of how colourless a diamond is. According to GIA, "diamonds are colour-graded by comparing them to stones of known colour under controlled lighting and precise viewing conditions".
Sapphires come in every colour you can imagine. Every single sapphire has its own unique colour. And describing this colour is not a simple task. The colour characteristics of sapphire can be further broken down into three colour properties; hue, saturation, and tone.
Hue is simply the obvious colour difference you see in the stone. There are numerous naming conventions to identify different hues such as navy blue, deep blue, sky blue, denim blue etc. However, there is no consistency in these naming conventions.
Saturation is how well the colour is distributed across the body of the gemstone and how vibrant the colour is expressed. The best stones have a vivid saturation and the opposite is a dull saturation.
Tone is the relative lightness or darkness of the colour. For example, two sapphires can have the same colour but one can be lighter and another can be darker based on how much grey colour is mixed with its main hue.