There used to be a rule in BE that Intermediate level and above was black or blue jacket, and Novice, BE100 and BE90 was tweed jacket up until a few years ago when the rules were relaxed to allow any jacket at any level. I believe its traditional for a tweed jacket to be worn with a coloured, spotted or non-white stock eg cream check pattern and a white stock to be worn with the black or blue coat. I think still at the lower levels tweed is more common as traditionalists stick to the old rules! Hope this helps.