Semi vs demi vs permanent at a glance
| Semi-permanent | Demi-permanent | Permanent | |
|---|---|---|---|
| Developer | None | Low volume | Usually 20 vol or more |
| Lifts (lightens) | No | No, or barely | Yes, a few levels |
| Grey coverage | Stains, translucent | Blends | Full |
| Lasts | Several washes to a few weeks | A few weeks, fading gradually | Grows out; tone still fades |
| Regrowth line | Soft or none | Soft | Visible |
| Patch test | Follow pack; often advised | Yes, 48 hours before | Yes, 48 hours before |
Semi-permanent hair colour
Semi colours use pre-formed dye molecules that sit mostly on and just under the hair surface. There's no developer, so they can't lighten, and they wash out gradually. They're good for trying a tone, adding vibrancy to lightened hair, and fashion shades such as pastels and brights. On light or porous hair they can last longer than expected.
Demi-permanent hair colour
Demi colours are mixed with a low-volume developer. The small dye molecules develop inside the hair, so they last longer than semi colour, but they don't lift natural pigment much. Many are ammonia-free or low-ammonia, though they may use other alkalising agents. They're the usual choice for toning, glossing, darkening, and blending grey.
Permanent hair colour
Permanent colour contains an alkaline agent (often ammonia or an alternative such as ethanolamine) and is mixed with developer. It opens the hair, lightens some natural pigment and develops new colour inside the hair at the same time, which is how it can lift and cover grey fully. The colour doesn't wash out, though its tone can still fade, and it grows out with your hair. For covering grey specifically, see covering grey hair.
PPD and hair dye allergy
Most permanent and many demi-permanent dyes are oxidative, and many of them use para-phenylenediamine (PPD) or toluene-2,5-diamine (PTD) to build darker shades. These are known to cause allergic reactions in some people, and sensitivity can develop after years of trouble-free use. That's why brands ask for a patch test 48 hours before every application. Products labelled "PPD-free" may still contain related chemicals, so read the ingredient list and still patch test.
Safety first. Do an allergy patch test 48 hours before every permanent or demi-permanent colour, even a brand you have used before, and a strand test to check the result. Many oxidative dyes contain PPD (para-phenylenediamine) or related chemicals that can cause serious allergic reactions. This site gives general information, not medical advice: if you react, rinse the product off and get medical help.
Which type of hair dye should you choose?
- To try a new tone with no commitment, use semi.
- To tone lightened hair, add shine, go a little darker or blend a little grey, use demi.
- To go lighter by a level or two, or cover a lot of grey, use permanent.
- To go much lighter, use lightener and then toner. See going lighter.
Dye type questions
Is demi-permanent colour less damaging than permanent?
Demi uses a low-volume developer and doesn't lift much natural pigment, so it is usually gentler. It still needs a patch test 48 hours before.
Does semi-permanent dye need a patch test?
Follow the pack. A patch test is often advised, and it is required for demi and permanent colour.
Is PPD-free hair dye safe for people with allergies?
Not necessarily. Products labelled PPD-free may contain related chemicals, so read the ingredient list and still patch test.