The present invention is drawn to an agent and process for altering the color of keratin fibers wherein the agent comprises: (A) a color base composition containing a fatty substance having a viscosity greater than about 50 mm2/s at 40°C, a rheology modifying polymer; an alkalizing agent; a short alkyl chain hydroxy compound chosen from monoalcohols and polyols; and an oxidative dye precursor; and (B) an oxidizing composition comprising an oxidizing agent.