I wouldnt worry about the color. For those dairy breeds at most fairs, as long as its slick sheared they dont care. Jerseys and Brown Swiss come in quite a mix of different colors, maybe black butt and the rest of her brown. You never know.
Changing coat colors and using different techniques on dairy cattle now is a huge no-no at most fairs. Check with where youre showing first to see if they allow it or you might get DQ.
On dairy you clip them all the way down and you want the topline to look as stright as possible. No hair to build toplines and no paint.