The present invention relates to a composition (A) for dyeing or lightening keratin fibres, in the form of a microemulsion comprising: i) at least water, ii) at least one oil, the total oil content being greater than or equal to 5% by weight relative to the weight of the composition iii) at least one surfactant, the total surfactant content being less than or equal to 19% by weight relative to the total weight of the composition and iv) at least one compound chosen from: a. colouring agents chosen from direct dyes and oxidation dyes, and/or mixtures thereof and b. oxidizing agents, c. basifying agents, and mixtures thereof, v) optionally at least one liquid organic compound with a Hansen solubility parameter value δH of greater than 0 and less than 16 MPa½. The present invention also relates to a process for dyeing or lightening keratin fibres, comprising the application of a composition A, optionally preceded by mixing with a composition B comprising at least one oxidizing agent, and also to a multi-compartment device and to a ready-to-use composition resulting from the mixing of compositions (A) and (B). The composition according to the invention makes it possible to lighten or to dye keratin fibres strongly, efficiently and homogeneously while at the same time being stable and easy to apply.La présente invention concerne une composition (A) permettant de colorer ou déclaircir des fibres de kératine, sous la forme dune microémulsion comprenant : i) au moins de leau ii) au moins une huile, la teneur totale en huile étant supérieure ou égale à 5 % en poids par rapport au poids de la composition iii) au moins un tensioactif, la teneur totale en tensioactif étant inférieure ou égale à 19 % en poids par rapport au poids total de la composition et iv) au moins un composé choisi parmi : a. des agents colorants choisis parmi des colorants directs et des colorants doxydation, et/ou des mélanges de ceux-ci et b. des agents oxydants c. des agents basifiants, et des mélanges
