I have been diabetic for over 2 years, and have 31 tattoos, most of them done before I developed diabetes. There is absolutely no reason why you cannot have a tattoo if you are diabetic, you just need to take some extra precautions and your after-care must be meticulous. Why let diabetes stop you from your lifestyle? Diabetic nurses and doctors will tell you not to do it due to the time it takes to heal and the risk of infection - however if you are careful with your after care there should be no problem.
Firstly, make sure you have eaten adequately on the day, and take something with you to both eat and drink in case your sugar level drops during or after the tattoo. If it is your first tattoo and is rather large, you may develop "tattoo shock" which can lower your blood sugar level. The tattooist will put a dressing on your tattoo when it is done - keep this on for a few hours, then take it off, clean it thoroughly and apply a thin layer of Savlon.
Secondly, when it is healing, KEEP IT CLEAN! Always wash hands before and after cleaning and caring for your tattoo. A mild soap will take care of the cleaning, use Bepanthen or Kamillosan cream on the tattoo once it is dry - these are actually nappy rash treatments, can be found in your local supermarket and work a treat on healing tattoos.
As for the healing process, colour cannot become "sucked out" of a tattoo - if you dont use the cream adequatly, your tattoo will form large scabs, and when they fall off they will scar your skin - you cannot see through the scar, so it looks like the colour has come out - it is still there, just under the layer of scarring. If this does occur, you can always let it heal and go back for touch-up work later.
If your new tattoo starts to swell, get increasingly hot and painful, go straight to the hospital - infection can cause septicaemia if not caught early enough. However, if you keep it clean enough, this will not happen.
I have been having tattoos since I was 18. I'm now 41, and am going for my 32nd tattoo on Saturday. I refuse to let diabetes get in the way of anything I want to do, and neither should you.