Image plays a big part in your success especially if your after the retail market.
Everyone seems to have their own idea of what is proper though. I wear a grey t-shirt with the company name and a one line description of what we do across the front. My new partner asked me why we don't have phone numbers on our shirts and I told him it's because I don't try to use my shirt as an ad tool. I've thought about putting a catchy slogan or some phrase like "Ask me what I can do for your vehicle", but I don't see the point in having your phone number on there though unless it is catchy.
A fellow mobile detailer in my area wears a black t-shirt with bright yellow lettering and he believes that is best for his business.

Working in a black shirt in the Florida summer sun is a big mistake if you ask me but he is sold on the idea.
Speaking of black being a mistake IMO, there was a mobile detailing company in town for a couple years that used to use a black F250 Crew Cab and a big black enclosed trailer to work out of.
I always shook my head when I saw it because I knew he had to spend so much time trying to keep his set-up looking nice and it just didn't make sense to me. I'm sure he saw it as a great advertising tool.