Permanent vs. Demi-Permanent

From what I've been taught and from experience here's a little insight on differences between permanent hair color and demi-permanent and the few others you may hear.

Permanent- Yes it's permanent it contains peroxide and ammonia which means it should be used only as directed to avoid damage or unwanted results. Permanent hair color covers grey, can lighten hair 1-3 (possibly 4) levels, or do both at the same time. I do not recommend putting permanent color over itself, this can create damage, darker, and/or darb ends. Lasts forever, can only be lightened with color remover or bleach.

Demi-Permanent- Shiny and typically amonia free hair color that can change the tone of your hair color, deepen/refresh color, can cover or blend to 50 percent grey, or can create a darker desired level while allowing hair to remain healthy. Won't lighten hair, and cannot create very drastic results on dark and darkest brown hair. Lasts 4 weeks to 3 months, can be used over itself without uneven results. Contains peroxide but not ammonia (better for your hair) You cannot lighten hair with permanent color over hair that has been previously color with demi.

Semi-Permanent- Color that simply lays on the hair shaft, most often used as punk/funky colors (ie. blues, greens, pinks and purples). Washes out in 6-12 shampoos or 1-3 weeks.