Callisto, depends on the level. At your local show a jockey skull with velvet cover would be OK if you were only doing one or two a year. You'd wear this for Pony Club as well.
However for reasonable quality showing or if you were really keen then a Charles Owen velvet hat with discreet chin strap would be the way to go.
DD rides her M&Ms in brown hat, brown checked jacket, buff jods, white shirt, orange tie and either orange ribbons or false bun with orange scrunch depending on the class.
She rides plaited ponies (ie show ponies etc) in blue hat, blue jacket, yellower jods, white shirt, red tie with tie pin, red/gold ribbons.
Blue hat/blue jacket would also be correct if you qualify for an evening performance regardless of type of pony.
With both ponies she wears brown jod boots but she uses jod clips on the M&Ms but discreet elastic on the plaiteds.