People do not give ads or fliers much of their attention period, even if they are catchy and persuasive. This is why political signs are so simple: patriotic colors, last name, party, office, maybe a short slogan. People running for office and those who get them elected are the ultimate pragmatists in that regard, so it bears some translation to the business ad you are trying to craft.
I would say that once you have captured their glance (and really, it is a glance) then you have one sentence, perhaps two, to draw them in. Once they read the whole thing, then the odds of gaining their business increase dramatically. So choose a color that attracts the reader, and distinguishes it from other postings/handouts/inserts, then choose one sentence that encapsulates what you offer, and make it more noticeable than the rest.
"Why call an ambulance when there is no emergency?"
"Save money without sacrificing safety or quality of care"
"Medicab: Your safe, inexpensive alternative"