It does seem that every colour line has their own way of formulating for undertones or remaining pigment. I thought I would share a bit about how the undertones are created to help understand the lightening process more.
The Natural Pigments
There are two types of pigment present in all hair, eumelanin is a black or brown coloured pigment that is primarily responsible for depth and pheomelanin which is yellow or orange-red and is responsible for adding golden and red tones to the hair.
It is now generally thought that when the granules of melanin pigment are produced, they are produced in two steps whereby the
pheomelanin is first produced creating a core of yellow/red followed by a layer of brown/black eumelanin.
"The casing model"
Now it might seem obvious that red hair contains very high amounts of pheomelanin, and golden blonde hair also has more pheomelanin than ash-blonde hair BUT if the pigment granules are encased in a THICK layer of eumelanin this can mask the underlying pheomelanin, until you lighten the hair. This can explain why sometimes the hair might look flat, without warmth until you lighten it and tones of warmth is exposed!
Lightening the Hair
When you lighten the hair, the pigments are oxidised in two steps:
Firstly the pigment granules are solubilized by the peroxide, and then slowly dissolved. This initial reaction changes the colour of the eumelanin from black-brown to a reddish-brown. When you lighten dark hair, the undertone will generally be a red-brown tone due to the huge amounts of eumelanin being oxidized.
If you were to continue lightening the hair, and therefore removing the eumelanin, the once hidden pheomelanin becomes more visible, adding orange and yellow tones to the hair. The pheomelanin is more resistant to oxidation, therefore if you are creating a lot of orange when lightening the hair with hair colour, to remove it you would need to switch to using pre lightener which can break the pheomelanin down from orange to pale yellow.
Working with the undertone
There was a lot of discussion regarding if you neutralise that undertone of the natural colour or the desired level, and in my opinion they are both right!
For example if you were lightening the hair from a level 6 to an 8, the classic chart of undertones might tell you that the undertone is yellow- therefore a violet tone would be used to neutralise.
BUT if you can see in the light that there are orange tones visible in the base colour then this means that there is that underlying orange-pheomelanin which will be exposed when lightening so you should use a blue tone to neutralise.
However as explained earlier this is not always easy to see, especially if you are working on dark hair. In this case you can lighten a test piece of hair with a neutral or clear level 10 colour and observe the tones that are being exposed, where it is more yellow, orange or red and then you will know the best tone to neutralise for that individual's hair type.
The final consideration is how intense/saturated the undercoat is, for example the undercoat might be orange but how orange is it? If you were to put it into numbers would it be a soft copper or an intense copper? The intensity of the undercoat will tell you how much ash tone to add into your mixture.
Usually the coarser the hair, the more pigment it contains and therefore the more warmth will be exposed, so on coarse hair you might use a double ash shade, whereas on very fine hair a natural-ash might suffice.
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