To be correct (and it's much nicer anyway (MHO!) brown tack for all riding horses of any description or age. Black should only be used for heavy horses, carriage horses and the cavalry.
In some circles, black is considered as very cheap and inferior so go for class every time and use brown!
My gelding is dark bay and has brown tack, cant imagine him in black. My grey is in black tack, she has worn a brown saddle it looked ok but black suits her more.
Havanna- darkish brown tack - on every colour horse for me.
Even our black driving harness has dark brown reins
( stops any dye ruining brown driving gloves in wet weather)
black all the way! I reckon it looks nicer and its much easier if you need to replace any bits of your bridle or get martingale etc - there is only one colour of black so it will always match!!