Primary and secondary colours

1Why this matters

Look around a Rwandan town: the blue, yellow and green of the national flag, shop fronts painted in bright company colours, kitenge on the market stalls, flowers at a wedding. Every one of those colours was chosen by someone, and most of them were mixed from just a few starting colours.

A painter who understands how colours are made can buy three or four tins and mix dozens of colours from them, instead of buying a tin of every colour. This lesson explains which colours are the starting points, what you get when you mix them, and the words used to describe any colour.

2How we see colour

White light, such as sunlight, is a mixture of all the colours of the rainbow. An object looks red because its surface reflects mainly red light into your eyes and absorbs most of the other colours. The coloured substances in paints, dyes and inks that do this absorbing are called pigments.

3Primary colours

Primary colours are the starting colours from which other colours are mixed. There are two systems, because mixing paint and mixing light work in opposite ways.

• Paint (pigments). The primaries taught for painting and decoration are red, yellow and blue. Each paint absorbs part of the light, so every paint you add takes more light away and the mixture gets darker. Mixing all three gives a dark brown-grey. (Printers use a more exact set of pigment primaries, cyan, magenta and yellow, plus black. Look inside a printer's ink cartridges.)
• Light. Phone and TV screens and coloured stage lamps use red, green and blue light. Each light you add gives the eye more light, so the mixture gets brighter. All three together make white light.

4Secondary colours

A secondary colour is made by mixing two primary colours in equal amounts.

Notice that the same word can belong to different families: yellow is a primary in paint but a secondary in light.

5Describing any colour: hue, value and intensity

• Hue is the colour's name: red, orange, green and so on.
• Value is how light or dark it is. Adding white makes a lighter tint; adding black makes a darker shade.
• Intensity (also called saturation) is how bright and pure it is. A traffic-light red has high intensity; a dusty brick red has low intensity.

Black, white, grey and most browns and beiges are called neutral colours. They are not primaries or secondaries, and they go with almost any other colour.

6Mixing paint in practice

• Real paints are not equally strong. Blue and red usually overpower yellow. So to mix green, start with the yellow and add blue a little at a time; to mix orange, start with the yellow and add red.
• Record a ratio. When you find the mix you like, write down how many spoonfuls or cups of each you used, for example 2 parts yellow to 1 part blue. Then you can mix the same colour again, in any quantity.
• Mix enough. It is very hard to match a hand-mixed colour a second time, so mix all you need for a job at once.
• Test and wait. Paint a small patch on a scrap and let it dry, because many paints look slightly different wet and dry.

Worked example: mixing colours for a class mural

An S1 class is painting a mural of Rwanda's hills under a bright sun. The school gives them red, yellow, blue, white and black poster paint.

(a) Which colours can they make? Green for the hills (yellow + blue), orange for the sunset (yellow + red), violet for the distant mountains (blue + red), light sky blue (blue + white, a tint), and dark green for the shadows under the trees (green + a little black, a shade).

(b) How much of each paint? They need 600 mL of green, and their test patch showed that 2 parts yellow to 1 part blue gives the green they want.

(c) The order of mixing. Pour the 400 mL of yellow first and stir in the blue gradually, because the blue is the stronger paint.

Practice

Solution. (a) Orange. (b) Yellow. The answers differ because paints absorb light while coloured lights add light together.

Solution. Violet: secondary (blue + red). Blue: primary. Grey: neutral. Green: secondary (yellow + blue).

Solution. Total parts = 3 + 1 = 4. One part = 900 ÷ 4 = 225 mL. Yellow = 3 × 225 = 675 mL; red = 225 mL. Check: 675 + 225 = 900 mL.

Solution. Add more yellow a little at a time, which moves the green back towards yellow and makes it lighter; or add some white, which makes a lighter tint of the green. Next time she should start with the yellow and add the blue gradually.

Solution. They differ in intensity: the traffic-light red is bright and pure (high intensity), while the brick red is dull (low intensity). The brick red may also be slightly darker in value.