There is no grey hair; there is only white hair. The grey effect comes from the mixture of white and still naturally pigmented hair.
If you want to know more about white hair read our chapter white hair.
How does it look if you paint a black and white wall blue? In most cases you’ll get a pattern of light blue and dark blue. In order to get an even result it is important to equalize the colors prior to painting. The stronger the difference on the wall, the more challenging it is to equalize the colors.
Grey hair is like a black and white wall - so that's why its a challenge to color it because both hairdressers and clients want an even color result with the white hair covered.
Trendy, fashion shades have a tendency to cover less than natural shades making the resulting color too vivid.
The white hair is shining through more intensely than the darker hair.
It’s important to equalize the white while coloring the rest of the hair. This can be done easily by adding N or NN shades to the chosen formula. Keep in mind that the larger the percentage of white hair is present, the more N-shade must be added.
The intermix of N-Shade (same level as fashion-shade) causes an adjustment of the color so that the color shows a more equal result.
Sometimes even the addition of an N-Shade doesn’t provide sufficient coverage of white hair. White or gray hair tends to be very resistant. It still shines through because it’s less covered. In these cases a special NN series is required for sufficient coverage and reliable results.
Resistant hair requires special treatment compared to normal hair because of its structure. The conventional alkalinity and normal amount of dyestuffs does not provide sufficient gray coverage.
A stronger alkalinity of special grey products is needed to open the cuticle of resistant hair and to ensure better penetration of color pigments.
A higher concentration of dyes is needed to add the sufficient amount for gray coverage, so that enough color pigments ensure an adequate color saturation on stronger and thicker hair types.
Learn more about hair texture here.