The guides behind the formulas
Every formula on this site is a recipe for one head. These guides are the reasoning above the recipes: how a colorist reads a level, what lightener and developer actually do, which toner cancels what, and the questions people ask before they book. Each one is built from brand technical guides and standard color theory, and each links down to the formulas and tools that do the job.
Hair color levels
Bleached hair levels, stage by stage, 1 to 10 Bleach does not add a color, it takes the dark pigment out and leaves whatever warmth sits underneath. That warmth runs in a fixed order, red at the dark end and pale yellow at the light end, so the shade in the bowl at any moment is a reading of the level the hair has reached. Read the guide 
How to fix orange hair after bleaching gone wrong Orange is a stage, not a verdict: it is the one every dark head passes through on the way to blonde, and the bleach stopped there. The fix depends on two things you can check in five minutes: which orange you have, and how much more the hair can take. Read the guide 
What level is my hair? Read it at the root, the way a colorist does Hair level is how light or dark hair is, 1 black to 10 lightest blonde, and it is a separate thing from the tone. Read yours on a dry piece from the crown, fanned out, against the ten level pictures. Colored hair has two levels, root and lengths, and bleached lengths read by their pigment: orange is about a 6 or 7, yellow an 8. Torn between two? Fan the piece out and, if it is still close, take the darker one. Read the guide