An apparatus for coloring optical fibers, includes: a first reservoir for containing a natural coating material to be applied onto an optical fiber being drawn; a second reservoir for containing a colorant to be mixed with the natural coating material; a mixer in fluid communication with the first and second reservoirs to receive a first flow of natural coating material and a second flow of colorant and to mix the first and second flows to obtain a colored coating material; a coating die in fluid communication with the mixer to receive the flow of colored coating material and to apply it onto the optical fiber being drawn. A first gas pressure source is associated with the first reservoir, exerting on the natural coating material a first gas pressure variable as a function of at least one optical fiber drawing parameter, and a second gas pressure source is associated with the second reservoir, exerting on the colorant a second gas pressure variable as a function of at least one optical fiber drawing parameter. The first and second lines have relevant pressure drop in a ratio corresponding to the ratio of natural coating material and colorant providing the colored coating material.