My daughter has a bright bay with brown tack & she wears greeny tweed with navy collar & wears a navy hat. This combination looks really smart.
If you have a black velvet hat and don't want to shell out on a new blue one, then I'd go for a grey tweed (there are some very nice nice ones around if you get a good quality one).
If your hunting then really shouldn't be in a tweed anyway, only the juniors are in ratcatchers unless your cubbing, you can get away with it but its not correct.
Grey/blue tweed is rarely seen in show rings.....and I'm afraid a black hat for ladies is a no-no.
I'd recommend a greeny tweed (or a greeny brown with brighter overcheck) with red/burgundy tie and navy hat if you want to be correct.
And brown gloves/cane. Definitely not black as this is for mourning and is totally incorrect in the show ring.
Not blue tweed and not a light grey (there are various grey tweeds wich have the brown running through them, there not completely all grey but some will go with a black velvet hat) people don't always have the money to buy a new hat so you make the best out of what you have already got, basically there are fashions, at the moment your dark herringbone green/brown tweed is the 'in thing' at both local and county level but if your showing at local level (just in case you are) then your not going to get judged on the colour of your tweed jacket, so long as you match up and have smart all round appearance, your attire and your horses tack is all correct and your horse goes well, that is all you need to worry about.
I mean as long as I can wear my black jacket and black hat (showjumping) for this winters hunting, then I can wait a year before i need to get stuff for WH.