White is required for recognized shows, but you'd want more of a beige/tan for hunters. Beige is also good (here anyway) for non-recognized dressage shows.
Honestly, though, I'd probably go with something else for show breeches. If you're getting a white pair for recognized dressage shows then the plaid pocket would probably be covered by a coat, but if not then it wouldn't have the "classical" look. Same with schooling shows- I've always worn a nice polo or riding shirt neatly tucked into my breeches. If you go that route, then the pattern will be very noticeable. Probably not what you're going for. Personally, I would look for something of a similar style without any patterning in beige. If you decide to show in recognized dressage shows, then consider buying a separate pair of breeches. Others that have actually shown recently may have other opinions, though!