Colour is a rich and complex aspect to painting and a good place to start is the colour wheel. The colour wheel is conceptualized as a circle where the six primary and secondary colours are spaced evenly.
The three primary colours are red, yellow, and blue. Primary colours cannot be mixed. Think of them as your raw ingredients.
Secondary colours are orange, green, and purple. Combining the primary colours makes the secondary colours:
red + yellow = orange
yellow + blue = green
blue + red = purple
Complementary colours are two colours that are across from each other on the colour wheel. They combine to make gray.
red + green = gray
yellow + purple = gray
blue + orange = gray
When purchasing oil paint, I generally prefer to buy more primary colours rather than secondary/tertiary/convenience colours. Primary colours are red, blue, or yellow, and generally will have just one pigment listed. Convenience colours are premixed – for example, Gamblin’s Chromatic Black is quinacridone red and viridian, which, being complementary colours, grey each other down to black. By adding to your collection of primary colours, you add to your bank of ingredients which allows you to create a wider range of mixed colours.
Some colours, while not technically primary, can be considered primary because they are impossible or near impossible to mix. Viridian and phthalo green are an example of this. While green is not a primary colour (it can be made by mixing blue + yellow), the colour of viridian (PG18) or phthalo green (PG7) is very difficult to mix and may not behave the same as the pure pigment.
Brown can be considered a dark orange, and therefore is the complement to blue. A note on black: from a tube, black is often “dead” looking. It is quite easy to mix black using two dark complementary colours, and it will provide you with a much richer black that you have more control over. If you use black from a tube, add another colour (such as ultramarine blue or burnt umber) to liven it up and change the temperature.
Characteristics of Colour and Pigments
Colour has three properties: hue, chroma, and value. It also has a temperature, which is a relationship; that is, a comparison to another colour (not an inherent property of the colour). The pigment, ie. the paint, also has characteristics, such as opacity, lightfastness, and tinting strength.
Hue refers to the “name” of the colour. For example, yellow, pink, violet, etc. This tells you roughly where it is on the colour wheel and if it is a primary (red, yellow, blue), secondary (primary + primary), or tertiary (primary + secondary) colour.
Chroma refers to the saturation of a colour – how vibrant or how grey it is. A pigment’s chroma can be reduced relatively easily but if you want a higher chroma colour it is best to choose a higher chroma pigment. Chroma cannot be increased except with the addition of higher chroma colours. Black, white, and grey are low/no chroma colours.
Value refers to the lightness or darkness of a colour, for example where it is situated on a value scale.
Colour temperature is the relative visual warmth or coolness of a colour. It is used to describe the relationship between two colours. On the colour wheel in the simplest sense, orangey-reds are generally the warmest colours, and blueish-purples are generally the cooler colours. Think of warm colours as being closer to fire, and cool colours as being closer to ice.
Pigments have characteristics as well. A pigment can be opaque, semi-opaque, or transparent. Some pigments have a high tinting strength, meaning they are quite strong and will change a colour mixture with just a small amount of paint. Some have a lower tinting strength, meaning more paint is required to change a colour noticeably. This is determined just through practice. It is a combination of the inherent characteristic of the pigment, and the quality of the paint. Higher quality paint contains more pigment and less medium and tends to have a higher tinting strength. All of these characteristics need to be taken into account when mixing colour.