Nope, pale 'preferred' only not a rule. Wearing dark gloves to me is a way of hiding bad hands or becasue people don't want to pay for a pale set. Never written for a judge that has ever commented on it (unless ot say that a G&R wasn't clear)
This is what my neighbour (very experienced rider) told me- obv if your hands bob all over the place but are in dark gloves, the movement will be less obvious to the judge.
The BRC rule book for 2013 simple says 'Gloves must be worn' so I would guess so. Always worth looking a the rule book yourself though I have it in PDF so I can easily search for specifics (keep it on my phone too )