Corrective color and the law of color

Coloring is the single heaviest domain on the clinical exam, and the corrective questions are where it is won or lost. All of them run on two facts: which color cancels which, and what pigment is sitting underneath at the level you stopped at.

Primaries	Red, yellow, blue
Secondaries	Orange, green, violet
Tertiary	A primary mixed with the secondary beside it on the wheel
Blue cancels	Orange
Violet cancels	Yellow
Green cancels	Red
Underlying pigment as hair lifts	Red, red-orange, orange at 5–6, yellow-orange at 7, yellow at 8–9, pale yellow at 10
Color does not lift color	Artificial pigment must be removed first — no developer strength changes that
A soap cap	Shampoo, lightener and developer, for a small and gentle lift
Line of demarcation	The visible band left where a retouch overlapped

Where the point is lost: Two look-alike problems have completely different causes. Hair that turned brassy three weeks after a service is almost always a toner that faded, revealing the stage the lightener stopped at — lightening ended the moment it was rinsed. Hair that turned green after a summer of swimming is copper and other minerals deposited in porous hair, so it takes a chelating treatment, not a toner. Reaching for violet in either case makes things worse.