Tattoos that are gaining in popularity with hairstylists are usually the tools of the trade: scissors, blowdryers and combs. I have even seen a set of curlers, but to be honest, they kind of looked like a six pack of beer at first glance - so be careful if this is the direction you're headed. Barbers aren't without their fair share of work either. I've seen more than a few barber poles and straight razors, but also a particularly creative old-fashioned straight razor tattooed down the length of the barber's arm with the hinge at his elbow so it looked like the blade opened and closed. Some cosmetologists have done open scissors on their hands between their thumb and forefinger, so when they move their hands it looks like the scissors are cutting. (Be careful of hand and face tattoos - they can limit your job options because they can't easily be covered up with clothing in the workplace, so check with your employer or potential employers first.)
Cosmetology is a field that oftentimes likes to allow their purveyors to express the creativity through their physical appearance. While the standard clothing in a salon seems to usually be black business casual, the stylists often show their creativity through their hair color, style and sometimes their make up and nails. These days it seems as that a more permanent form of creativity is showing up on cosmetologists and it's in the form of tattoos. Quite a few cosmetologists are decorating themselves permanently with cosmetology tattoos these days with a common theme of showing off their passion and profession in the beauty business.