One of the dilemmas for us history fanatics (and Hollywood costumers, as well) is, how did city folk dress for posse duty? One school (the 30s and 40s Hollywood version) has the entire posse made up of cowboys. Or, at least they all wear cowboy duds, complete with chaps and tall crowned hats. The other version is to have the city guys wear pretty much what they wear in town (see Tombstone, the movie).
I think this classic C.S. Fly photo nails it:
This is Doc George Goodfellow of Tombstone fame, goofing for Buck (Fly) with a horse that Porfirio Diaz, the president of Mexico, allegedly presented to the doctor. The year is 1888.
Doc is dressed for a ride in the country. He has most assuredly exchanged his formal city hat for a big Sonora sombrero, adding riding gloves and canvas, or leather leggings. I think this is how the Earps, and their fellow townies probably dressed on their Vendetta Ride. Or, at least a variation of it. In other words, I think they adapted their normal city clothing, by adding a broader hat and various other accouttrements to fit the gig. Just a subtle difference, but a significant one to me.
"The Devil is in the details."
—Old Vaquero Saying