The invention relates (I) to a process for dyeing keratin materials, in particular keratin fibres, preferably human keratin fibres such as the hair, using i) indigo-producing plant powder, ii) at least one alkaline agent as a post-treatment, preferably an alkali metal or alkaline-earth metal (bi) carbonate and iii) optionally a chemical oxidizing agent such as hydrogen peroxide or a hydrogen peroxide-generating system in co-treatment with i), and it being understood that the composition comprising the alkaline agent(s) is at a pH inclusively between 7.5 and 11.5； and (II) to the use iii) of alkaline agent(s) for fixing the blue colour of and/or for improving the dyeing kinetics of keratin materials dyed with indigo.
