The following are attire guidelines in a nutshell, with the common practices being listed first, and the exact wording of the rules for each division--which is sometimes in archaic wording--below that:
In hunter and equitation classes, most people wear either navy, dark green, or gray jackets, all with or without narrow pinstripes (black is also acceptable, but rarely seen); light-gray or medium-gray breeches (although rust or canary breeches are sometimes worn); black dress boots or black field boots, with the boot tabs, toe caps, and spur rests all being optional (although brown is the traditionally correct color for field boots); white or pastel-colored shirts, with pale pink and blue seeming to be favorites; a black, velvet-covered helmet with either a leather or clear plastic chin strap (or a brown helmet if it is to be worn with a brown field boot); dark leather gloves (avoid leather imitations at all cost, for they usually slip when wet with rain or perspiration); very sheer hairnets for females, with the net matching the rider's haircolor as closely as possible (these can be obtained from most drug stores); black riding crops and spur straps to match black boots, or brown riding crops and spur straps to match brown boots.
In the jumper division, you often see white breeches, but the other attire is the same except in classes offering more than $25,000. In these classes, "only black, dark blue, dark green, or scarlet coats are permitted."
In equitation classes, the wording in the section on attire, referred to as "appointments," is as follows: "Exhibitors and judges should bear in mind that at all times entries are being judged on ability rather than on personal attire. Riders should wear coats of any tweed or Melton for hunting (conservative wash jackets in season), breeches or jodhpurs and boots. Conservative colored protective headgear in accordance with Article 318.3 is mandatory. Spurs, crops, or bats are optional. Judges may penalize contestants who do not conform." (Note: Article 318.3 deals with riders wearing a harness--i.e., a chinstrap--in order to secure their helmets on their heads.)
In the hunter division rules, the only references to attire are to "formal attire" and "ladies side saddle attire," so the restrictions in the equitation division (especially the references to "conservative" clothing) have usually been applied. The "formal attire" rules for hunter classics and appointments classes are as follows: "Riders are required to wear scarlet or dark coats; white shirts with white stock; white, buff, or canary breeches and hunting cap, derby, or hunting silk hat. Junior riders must wear protective headgear in accordance with article 318.3."