The level system in haircoloring measures:

In haircoloring, the level system measures the lightness or darkness of a hair color, typically on a numbered scale from darkest to lightest (for example, low numbers for dark shades up to high numbers for the lightest blondes). Level describes depth only, separate from tone (the warmth or coolness of the color). Understanding level — and distinguishing it from tone — helps cosmetologists analyze natural color and formulate or choose target shades. The level concept is a standard part of haircoloring theory on the exam.