Colour Theory for Hair Dyeing

Primary Colours
Colours are made up of one or more of the three primary colours red, yellow and blue with different amounts of each colour creating the resulting shade.
[Primary Colours: Red, Blue, Yellow]

Secondary Colours
These are made up of a combination of 2 Primary Colours. You can work out the secondary colours by finding the midpoint between 2 primary colours on the colour wheel. So, for example, the colour directly between red and blue on our colour wheel is violet. The colour between red and yellow is orange and the colour between yellow and blue is green.
[Secondary Colours: Violet, Green, Orange]

Tertiary Colours
These are made of a combination of a primary with a secondary colour. Looking at the colour wheel you can work out that Yellow-Orange is a tertiary colour since it's between a primary colour (yellow) and a secondary colour (orange). You can keep on segmenting the colour wheel but this is enough detail for the purposes of explaining hair dye.
[Tertiary Colours: Red-Orange, Red-Violet, Blue-Violet, Blue-Green, Yellow-Green, Yellow-Orange]

Complimentary (Neutralising) Colours
When combined these colours usually produce brown. They come in pairs of opposites on the colour wheel so red & green, blue & orange, yellow & violet. These colours balance each other out and can be used to correct dyeing mistakes. For example if a colour has come out brassy, ie unwanted red tones, a green based colour such as ash blond can be applied to neutralise the red tones. In the same way violet based toners are used to neutralise the yellowness in bleached hair to achieve a more neutral shade. The depth of the colour must also be considered. The two colours must be of equal strength to balance each other. Let's take the violet toner for blond hair example. Use too deep a shade of violet and the hair will be tinted purple. Use too light a shade and the hair will still have a yellow tinge. It's usually better to err on the side of caution and go for the paler toner as it's easier to add colour than to take it away. Also be aware that mixing two complimentary colours such as red and green will produce brown.
[Complimentary Colours: Yellow & Violet, Blue & Orange, Red & Green]

More Uses for the Colour Wheel and Hair Dyeing

Moving from one colour to another
A friend of mine currently has bright red hair but wants to eventually change her hair colour to green. She could let the red wash out and then dye over that with green but chances are that there would be so much red left over that the result would either be brown or greenish brown. A more gradual change would most likely result in a better final colour. It's much easier to cover one colour with another that is close to it on the colour wheel than to cover one colour with its complimentary colour. A faded blue can be easily dyed over with purple.

Take a look at the colour wheel on the left. It shows the starting colour of red and the desired colour forest green (indicated with a circle). You can see that these two colours are almost opposite on the colour wheel. You can see that if you want to gradually move from red to green there are 2 routes you can take:
Red - Red Orange - Yellow Orange - Orange - Yellow - Yellow Green - Green (clockwise arrow)
-or-
Red - Red Violet -Violet - Blue Violet - Blue - Blue Green - Green (anti-clockwise arrow)

There are 6 steps in either route so which is best? It all depends on the shade of green that is required. Since my friend wants a forest green, which has blue tones in it, it would be easier to go through violet and blue to reach green. If she had wanted spring green the other route would have been better.

Using the colour wheel to choose good combos
Some colours go with pretty much anything such as white and black but combining other colours can be a bit tricky.
The highest contrast can be achieved using colours at the opposite sides of the colour wheel (red/green, yellow/violet, blue/orange or Red-Orange/Blue-Green, Red-Violet/Yellow-Green, Blue-Violet/Yellow-Orange). These combinations will definitely make an impact but you'll have to take into account that if these colours bleed together they will produce brown. They're also pretty hard to look at (ever seen a magazine advert where there's red text on a green background and it appears to flicker?) so you might find you get tired of the colours pretty quickly.

You can get good contrast with combinations of secondary colours, however there is still the danger of getting brown when they're mixed but there are ways around this problem. In Adam's Adventures 16 we used 2 secondary colours - green and purple. To avoid an unwanted muddy tinges where the colours crossed, their common colour blue was used in between the green and purple. Since purple is made up of blue and red, and green is made up of blue and yellow, blue was the ideal colour to use as a buffer between the green and purple.

You can always attain great combinations when you have two primary colours and the secondary colour between them. For example when red and yellow are mixed orange is produced, so if you dyed your hair yellow fading to red putting orange in the middle will help the transition. Chunks of these three colours also look good together. You could also try yellow - green - blue or blue - violet - red.

Using colours to create depth
By choosing 4 or 5 colours from the same range you can create tonal depth to streaks and chunks. Start with one colour - for this example we'll choose violet. By mixing it in various proportions with a colour next to it on the colour wheel, blue or red, you can get a good range of colours for streaking hair, creating high-lights and low-lights. For a more adventurous look choose shades from either side of your base colour to mix with it in varying quantities. So with violet you could mix it with blue and with red to produce shades of blue violet and red violet along with violet. If you include blue and red streaks it will ruin the effect.