Dress for In hand Mountain and Moorland Showing?

This weekend I am showing a Grey (White) Welsh Section A 11.2hh. What can I wear and what can't I wear? Im 16 years old (I know there are some different rules for being over 16 which is why I put that in) about 5'6. I know I have to wear tweed but I was researching on a website that said something about white halters? I haven't got one but just wondering would it be correct to put one on a white pony? The pony is 19years old. Please help! thanks :)