Rules articulated here with which I both agree and have violated with, to my eye, good effect:
No navy trousers with odd jackets. Navy gabardine trousers worn with off white linen jacket (12 oz. broken herringbone), white poplin spread collar shirt worn open, navy with white dots silk square, dark brown suede tassel loafers and belt, no socks.
No black shoes with khaki pants. Khakis worn with black horse bit loafers, black calf belt, no socks, blackwatch tartan spread collar shirt, black silk knit tie, black and white herringbone tweed sport coat, green/red/yellow/blue/black paisley silk square.