I suspect the colour difference between near side and off horses was for handling.

However as the near and off horses were changed between near and off side, so the horses did not become 'one sided' when worked in the pair, as well as in a six horse team swapped between centre and lead horses. Wheelers could not be swapped with lead horses, as in theory they were larger horses and their harness was specifically for the wheelers. How often this changing of horses occurred in practice as opposed to text book theory I know not. It was meant to happen, but finding evidence of it occurring, which if it was a SOP, I don't think we will.

Parade head collars were supposed to be white leather, campaign items in Natural Leather, and ordinary service in Black. The use of the London Tan head collars can be documented in some period prints, eg Otto, and from stores return. Three head collars seems to me to be overkill. LIkewise not using the head collar as the snaffle bridle in British service, seems to me to uneccesarily clutter the horses head.

British harness was supposed to be all made from brown leather, in modern terms Havana. But of course London Tan leather can look brown.