I'm starting up lessons as an adult, and when I got my helmet (decided to buy my own), I decided to spiff it up a little bit. Car decals are perfect for a helmet, by the way! They are a little tricky to apply since a helmet is rounded, but in the end, it worked out great.
Here is my helmet! Definitely won't be mixing it up with anyone else any time soon if I accidentally forget it at the barn (happened once when I was a teen taking lessons)!