part one is what DR wrote. Altough can be done easier by preshading the lines with a dark color, then just use the needed color to paint all of it in one go (because of transparency the black preshade will show).Then you just need to make the larger panels lighter by airbrushing their middle part.
Part two is using an enamel ink to roughly paint in the panel lines, then using a solvent (white spirit for example) to wipe it off from the surface while leaving it in the line. This second part takes a bit of experienceto use correctly but is quite a fun process afterwards.
an interesting alternative could be using dry pigments for the shading part and after varnishing (to fix them in their place) have a go with the second part (because pigments won't be dark enough for the effect)