UK HSE Restriction Report Proposal for Tattoo and PMU Ink
The UK REACH Agency has released its restriction report for tattoo and PMU Inks, it's close to 300 pages and is pretty much in line with the EU REACH restrictions.

A brief overview of complications can occur in tattooing
Not all complications in tattooing are reactions to tattoo ink. Having knowledge of what you're dealing with will help to identify what the possible complication is and possible treatments (although this advice should be left to Dermatologists). We've linked to a brief article which goes through few different complications that can occur in tattooing and possible treatments that are sometimes used (sourced from the Tattoo Police, an outpatient clinic in the Netherlands)