In my opinion, at the bare minimum a roast needs a green vegetable (cabbage, broccoli, peas) , an orange vegetable (carrots, mashed swede) and a white one (potato, parsnips, cauliflower. Cauliflower cheese with ham/gammon only).
It does not need a yellow one.
I once got served baby corn with a Christmas dinner which was odd. The rest of the meal was lovely, but it just seemed a rather unusual addition.