The target zone
The epidermis constantly renews and sheds, so pigment left only there disappears within weeks. The deep reticular dermis is where body tattoos live (roughly 1–2 mm); go that deep in a lip and the rich blood supply and loose tissue spread the particles into a soft, dark, ill-defined stain.
The sweet spot for cosmetic lip work is the papillary to upper reticular dermis — deep enough to be retained, shallow enough to stay crisp and to fade cleanly. In practice that is a fraction of a millimetre, controlled by needle projection, angle and hand pressure.
Reading the skin
Experienced artists judge depth by feedback, not a depth gauge: the sound and drag of the machine, the colour of the fluid (clear lymph = good; heavy blood = too deep), how the skin 'accepts' the pigment, and minimal pinpoint bleeding.
Lips vary across their surface — thinner near the border, plumper centrally — so depth is adjusted continuously.
Consequences of getting it wrong
Too shallow: patchy heal, colour 'gone' after peeling, frustrated client. Too deep: migration, blowout, cool or grey healed colour, and pigment that is hard to correct or remove.
