Raul asks, "Why did the Earps wear all black suits? Was that the style of the day?"

Yes, as a matter of fact, it was.  Doc Holliday preferred to dress in grays, but the Earps preferred black.

There may be reasons other than plain fashion, though, why men of the time were so often seen in one color suit:  it may have been the only suit they had (bringing it out only when going to town or for special occassions), it may have been for convenience (after tearing one's pants, getting another pair of the same color preserved the full suit), or it simply may have been their favorite color.