Answer:
Every color has a degree of light or darkness which is caused the level.
In hair coloring the levels/numbers are from 1 to 10.
1 Is the darkest and 10 is the lightest.
1 would be for black and 10 for lightest blonde everything else in between is the intensity of the hair color.
1 Black
2Dark Brown
3Medium Brown
4 Light Brown
5Lightest Brown
and so on.
This also helps you determine your natural hair color.