Colorants in foam form

An agent in foam form for coloring keratin-containing fibers, in particular human hair, under induction by atmospheric oxygen and methods for coloring keratin-containing fibers using such agent is provided. In an embodiment, the agent comprises, in a cosmetically acceptable carrier, at an alkaline pH of 8 to 10.5, (a) as a color-forming component in the form of a dye precursor of a nature-analogous dye an indole and/or indoline derivative, (b) an alkalizing agent, and (c) a propellant, the agent containing no additional oxidizing agent for the color-forming components.