To achieve full color saturation, the needle of the tattoo gun must penetrate the epidermis and continue deep into the dermis. However, the needle must not push so deeply into the dermis that the tissue tears because this causes the formation of scar tissue, making the ink appear less vibrant.
Under-saturation occurs when the pigment is not pushed deep enough into the dermis, or when only a small amount of ink is applied. Tattoos that are under-saturated can be made more vibrant via a second application of ink. It is not harmful to tattoo over an already tattooed area of skin as long as the skin has healed from the first application of ink.