|Royal Rider stirrups|
My understanding of the reasoning is that a black iron against a black field boot, especially on a dark horse, can hide minor flaws in the leg position and make it difficult to tell if the rider has their foot in the stirrup.
If you are in the market for a new pair of stirrup irons and think that you might enter any equitation classes in the future, you should consider either a stainless steel-type iron or a newer-style polymer iron that is coloured silver/grey rather than black.
As far as I am aware, jointed stirrups with black joint coverings are most likely still acceptable because the top and bottom of the iron are still visible as stainless steel. If you are worried about potential elimination for those black coverings, the stirrups are made with grey coverings as well.
The colour of the stirrup pad should not matter because it is hidden by the rider's boot and the branch of the stirrup, anyway.
If you already own black stirrup irons and cannot purchase a new pair, there are metallic silver spray paints available that can be used on plastic.
I have not heard anything yet about this rule being adopted in Canada, but I would not be surprised if it happens in the next few years if this is a common complaint with judges, given how often American judges are brought up to Canada. For that reason, it might be wise for any equitation riders who plan on purchasing new stirrup irons in the near future to stick to stainless steel-looking options.
This is the new USEF rule:
EQ110 Appointments [Chapter EQ - Equitation Division, Subchapter EQ-2 Hunter Seat Equitation Section] add new:
4. The use of black stirrups is not permitted in any Equitation class. The use of black stirrups will automatically result in elimination.
The full wording of the rule change can be seen here (on page 169):