Identifying skin tone correctly is the foundation of the laser hair removal process.
The Knue Scale uses 10 shades, from Very Fair through Deepest Brown. You should be able to look at a client, place them on that scale, and immediately understand what that means for your wavelength, your settings, and your approach.
Sun exposure, self-tanner, lighting, and hormonal changes can all affect how the skin presents. Skin presentation can change, and previous assessment should not be relied on at the time of treatment.