Which part of the skin is injected with dye for tattoos?

The ink must penetrate the 3rd layer of the skin.

Well, I heard it like this! There are three layers of skin, the epidermis, the dermis, and the subcutaneous,or the Fatty layer. The top two must be penetrated for the tattoo to stay...Whether the third layer is touched doesn't matter unless you flood it with two much ink which will cause staining on the surface of the skin in and around the tattoo. Hope this helps.....