If visibleNorth on a face tattoo is set to false, body tattoos will also not be shown on the backs of colonists. (the face setting seems to override the body setting)
Also, all tattoos make colonists translucent wherever is masked by the tattoo; you can see what's behind them.
I've attached images with both, the only mods active are XML mods, Harmony and HugsLib, (as well as camera plus to better show the transparency issue) which I doubt will impact this.
Edit: Posted the wrong file for the translucency issue; the second one is correct.