With this basic background in colour, we can begin a more advanced discussion of colour as it occurs/appears in the face-up direction of any natural coloured diamond, with confidence.
When colour diamonds are concerned the expression of colour is a unique, and quite confusing to most people, because it is the result of purposeful manipulation (i.e. cut and polish) of the diamond to produce an exceedingly complex mosaic of multitude of colours which derive from the natural inherent body colour of the diamond. The typical mosaic we know as what polished diamonds look like is a far-cry light-show display which we consider much more magnificent than the expression of the colour in the rough diamond.
The flat, opaque colour of a paint colour sample chip is distinctly simpler and vastly different from the complex, dynamic expression of colours (i.e., variety of tones of a given hue) which are seen in the face-up direction of a transparent, coloured diamond. While it is called “face-up colour” (singular), what we see when viewing a transparent polished coloured diamond in a “normal” (i.e. overhead) position, with light striking from above is that complex mosaic of reflected and refracted multitude of colours, which we attempt to “sum-up” into a definition that answers the all important question: What is the colour of this diamond?