On natural or 100% grey/white hair, the colour is not affected at all. On coloured hair, the treatment may lift it by 1 to 1.5 levels. On partially lightened hair (highlights, balayage), the treatment may cause some lightening, but the violet pigments in the formula will offset it. It’s very important to follow the flat iron temperature guidelines on the chart in order to minimise the loss and/or potential over-oxidation of colour.