Since tattoo ink is deposited into the dermis layer (below the epidermis), one can say that it is actually below the skin - but actually just below the first skin layer.  This is only about 1mm below the surface so for most 3D CG it would probably be a bit too much to attempt that level of detail.

But if you are looking for 'truly authentic and realistic' at close range, then an overlay of the skin texture with a particular amount of transparency should do nicely.  Not sure how all of this would affect sub-surface scattering (as tattoos are sub-surface).  As noted, 'surface' irregularities in the skin are still present (bumps, wrinkles, pores).

At any distance from the skin, getting too complex would be overkill.  Follow bagginsbills method and you'll be okay.  But definitely consider references and the age of the tattoo.

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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