Coloration and Lightening
Core concepts
Know the hair colour wheel and law of colour: primary, secondary and tertiary colours; complementary colours neutralise unwanted tones (blue neutralises orange, violet neutralises yellow, green neutralises red). Understand depth/level (1 black to 10 lightest blonde) and tone, and how to read a shade number such as 6.43 (level 6, copper-gold tone).
Know developer (hydrogen peroxide) strengths and their lift: commonly 3% (10 vol) deposit only, 6% (20 vol) standard lift, 9% (30 vol) more lift, 12% (40 vol) maximum lift. Understand that lightening removes natural pigment and reveals underlying pigment (red, orange, yellow), and that the result depends on starting level, developer, and timing.
Know the difference between permanent, demi-permanent and semi-permanent colour: permanents lift and deposit and need a patch test; demis deposit with low or no lift; semis are direct dyes. Know strand testing, patch testing, and safe mixing (never metal bowls for peroxide, always follow manufacturer ratios).
Traps
Questions often mix up developer strengths or claim a higher volume always gives a better result. Another trap is forgetting that lightening exposes warm underlying pigment.
Rule of thumb
Match developer to the goal: deposit needs low volume; lift needs higher volume, but never exceed manufacturer guidance.