Hi and welcome to the forum and to BSA, B31 nice bike. It seems that the silver paint colour on the tank is correct. According to the book BSA Gold Star and other singles by Roy Bacon the colours for 1946 are " Chrome plated tank with matt silver panels lined in black. Winged BSA badge on tank side. Wheel rims matt silver with black lining. Brake plates chrome plated. frame, forks, headlamp shell, mudguards, oil tank, toolbox, chaincase and chain guard all black." All the best with the project.