Rewritten September 2026. The test below was carried out in 2024.
The short answer: mix plant hair colour with water, and nothing in place of it.
Oil alone gave no colour at all in our test. Water with some oil worked, but the result was lighter than water alone. Oil does not release the dye, and it does not help it reach the hair.
“Can I mix olive oil into the paste?” and “Can I use coconut oil instead of water?” are two of the questions we are asked most often. The reason is usually the same: plant colour can leave hair feeling rough, and oil seems like the obvious fix. Rather than answer from theory, we tested it.
What we tested
We mixed the same colour three ways and applied each to a sample of grey human hair, leaving them on for the same time.
What happened
Why oil alone gives no colour
The dye in a plant colour is not sitting in the powder ready to use. In henna leaf, the pigment lawsone is stored bound to sugars, as compounds called hennosides. In indigo leaf, the pigment starts as a colourless compound called indican. Both have to be split by enzymes in the leaf before any colour is available, and that reaction happens in water.
With oil and no water, it does not happen. The pigment stays locked in the plant material, which is why the oil-only sample came out pale.
Why adding oil made the colour lighter
We did not measure this directly, so these are the likely reasons rather than findings. Replacing part of the water with oil leaves less water to release the dye from the same amount of powder. And oil is good at coating hair: some of it will sit between the paste and the strand, where it is in the way of the pigment. It is the same reason we ask you to wash out oils and conditioner before colouring.
What this means when you mix
Use water, and nothing in place of it
Mix with warm water only. If you have been adding oil for a smoother paste, the paste will spread well enough without it, and your colour will be deeper. How to mix and apply →
Start with clean hair
Oil already on the hair has the same effect as oil in the bowl. Wash out hair oils, serums and conditioner before you colour. Eight reasons natural dyes do not dye →
If you want oil, use it afterwards
Oil or conditioner after rinsing will not affect colour that has already taken. With shades that contain indigo, wait until the colour has finished developing, usually two to three days, before adding anything to the hair.
About the rough feel
If you were reaching for oil because your hair felt rough after colouring, that feeling is common and usually temporary, and there is more to it than dryness. We cover what causes it, and what helps, on a separate page. Why hair feels rough after plant colour →
What this test does and does not show
This was a small test: one shade, one oil, and one sample of human hair for each paste, photographed three days after colouring so the indigo had time to develop fully. It shows clearly that oil alone gives no colour, and that oil in the mix gave a lighter result for Soft Black.
It does not tell us exactly how much oil is too much, how lighter shades behave, or whether coconut, argan or other oils behave differently. We expect them to, broadly, behave the same way, because the missing ingredient in every case is water, but we have not tested them.
Questions we are asked
Can I mix plant hair colour with olive oil instead of water? No. Without water the dye is not released, and in our test the hair did not colour at all.
Can I add a little oil to the paste? You can, but expect a lighter result. In our test, replacing about 30% of the water with olive oil, by weight, made the colour noticeably lighter than water alone, even after three days of development.
Does coconut oil work differently? We have not tested it. We would expect the same result, because it contains no water either.
Will oil stop my hair feeling rough afterwards? Not in the paste. Use oil or conditioner after rinsing, and with indigo shades, after the colour has developed. Why hair feels rough →
Should I oil my hair the night before colouring? No. Oil on the hair gets between the pigment and the strand. Colour freshly washed hair.