Why does Winsor & Newton refer to colour bias rather than Munsell rati

From time to time we are asked for an exact numbering system or chart regarding each of our colour's positions on a colour wheel, such as the Munsell system.

Munsell values, based on specific measures of hue, chroma and value, are judged from colour computer readings on the full strength mass tone of a colour in a draw down of controlled thickness - essentially a "single" colour. The benefit of this is that it provides continuity between colours and allows a colour to present a single rating.

However, we believe that artists’ are better served by looking at "colour bias”, which cannot always be determined straight from the tube. Understanding colour bias is crucial to successful colour mixing. To get the brightest mixtures, choose base colours with a similar bias, meaning those that “lean” toward each other on the colour wheel.

For example, although you mix yellow and blue to get green, which yellow mixed with which blue will produce the cleanest, brightest green? This is where colour bias comes in. To get the brightest green, choose a blue that leans toward yellow and a yellow that leans toward blue. This ensures the mixture contains only two colours: blue and yellow. If you were to choose a blue with a red cast instead of a yellow cast, you would introduce a third colour, red, to your mixture, and red, which is the opposite or complement of blue, will make the mixture muddy. Colour bias is sometimes also referred to as the colour temperature, i.e., is it “warm” or “cool” in tone.

In the example below, the greenest or cleanest green is made by using a green shade blue and a green shade yellow in Professional Watercolour: Ultramarine (Green Shade) and Cadmium Lemon. If a red shade blue, French Ultramarine and a red shade yellow, Cadmium Yellow Deep are used instead, the result is a dirty green.

To most accurately judge colour bias and select colours for your palette, it is best to consult a hand painted colour chart, which shows a gradation from mass tone to undertone for each colour. A colour’s bias is most easily seen in its undertone, which appears in a thinly applied layer of the colour.