I wish that I had an answer for you. I have died my poodle's tail and ears before, and have made stars on her for July 4th. For one I used chalk, the other I used blo pens (like stenciling). I know that there are dyes made specifically for dogs, so if I was you, I'd stick with those, just to be safe.
I have to say, you've been given a very hard time on this topic. I'm not sure why everyone thinks that because you're a groomer you would know everything there is to know about dyeing a dog. It's not something that standardly done to clients dogs so, unless there's a special interest why would you know? They don't realize that there's alot in grooming, and that there is so much to learn. Its not just cutting hair.
Have you seen some of the creative grooming competitions that groomers enter? They're incredible.
As far as why you would want to dye your dog. That's your business. In my opinion, its no different than putting a bandana, a fancy collar, or a dress on your dog. Products are meant to be safe, and just like doing a different hair cut, you want to be individual.
When you do it, I would love to see the pictures! Here's one I did of my poodle (stars)
http://i5.photobucket.com/albums/y193/fannemoran/bowsstars.jpg
Her poodles are black so I don't think dye would show up anyway.
