For showing it would always be either a navy or a tweed. For SJ/dressage I think it is more up to you, I prefer navy. Personally I would get a tweed, you can do anything in that.
Tweed definitely absolutely no question if you are thinking of showing your cob. Nothing worse than seeing black jackets in showing IMO. You can do everything with a tweed.
Well I don't show and I don't know much about showing so in that case I can't help you. But over the matter of years I bought one Navy jacket on sale and one Black on sale, as well as various colors of stocks and vests - so I can just pick and choose as I feel like on the day haha!
But get what you want to start with and then when you get some extra cash roll in look for a sale and buy another colour - no harm in having more then one jacket
They don't have to be expensive - ebay..... But you will find that at shows, most people will be in tweed - and those that aren't will be showing show ponies or hacks. Tweed is correct for Cobs, Riding horses, M&M and hunters.