The filters of opposite color will darken the tattoo. Filters of the same color will lighten it. Same goes for skin imperfections as holmsburgers has described. A yellow filter will slightly enhance a tattoo probably. Green might lighten it and bring out skin imperfections more (as it's would darken reddish blemishes and freckles) I would suggest bringing along filters depending on the tattoo color and trying some things.

If it's brightly lit, you might use slower film too, unless you're after a gritty look. I think skin might benefit from the smoother tones of a quality iso400ish film. I recently made nice handheld tmy2 photos with my yashica-c TLR of my daughter in the dentist's chair getting worked on, and I'm guessing room illumination at the tattoo shop would be brighter than that.