Knowing a few things about how hair colour actually works will help you better understand the options presented to you at your professional hair salon.
It all starts with the hair colour numbering system:
The first number is the base colour and indicates how light or dark the colour is.
1.0 is black, 5.0 is light brown, 6.0 is dark blonde, 7.0 is medium blonde and 10 is the lightest blonde.
After choosing the base of your colour, your colourist will look at the second number in the system that comes after the period mark.
.1 is Blue, .2 is Violet, .3 is Gold, .4 is Copper, .5 is Mahogany, .6 is Red, .7 is Mat, .8 is Mocha.
If your hair colour is with 2.7 it would be a dark brown with mat or green tones, and a 9.2 would be a light iridescent blonde.
This system also allows your colourist to neutralize the colouration, so to avoid a blonde hair colour from turning too warm, they could add more .2 - Violet or .1 - Blue to counteract the yellow or orange tones.