As far as helmets go, I'm a big Tipperary fan but for english, I'm always trying to find a helmet that has a slimmer profile b/c I wear a large and ALL helmets look funny on me - I like to look proportional if at all possible LOL! But you can always get a plain black helmet for arena work and get you a wild cover (tie dye, neon, whatever your fancy) for hacking out.
And very much a yes - conservative colors are in order for lessons but when you go out for a trail ride, ride in whatever colors make you feel good! But then I'm an endurance rider so I go loud when I get outside the fence - my 'conservative tack' (yes I do ride dressage as well and conform when it's necessary haha ) mostly collects dust in my trailer!!