Before the 19th century, both doctors and lawyers wore black coats. As time passed, the doctors changed the color of the coat to white to promote hygiene. The motivation to wear a black and white coat is the same. Both colors have their own meanings.

》Black is the colour of power and authority. Black also implies submission . priests wear black to signify submission to God . So is the case  with lawyers. their submissions are towards justice .

》Excepting the symbolic of white colour. There are more realistic reasons for choosing white coats. White coats are easily to get dirty by the blood and pus during the surgery .Germs are easy to be contained in these things. So white coats  can help doctors to detect the potential source of infection quickly.