Graham/Des
Well, it's not a foul. Not in and of itself.
There's no rule against additional colour so long as the players are still wearing, for the most part, their club colours -- so robes still required for play. That is, the political statement (in this case) can't overtake the uniform. Can't have people getting confused over who is on which team. And of course any changes should have ownership approval, though I can think of few owners who would fight their players on this in the current climate. Quite the hot button topic.
Closest thing we've found to a rule against additional colours is an old bylaw banning the Welsh from wearing a leek during play. It was overturned shortly thereafter. Never deny the Welsh their heritage.