I would color the woman in the center wearing the blue polka dot dress with a light blue or a pastel shade. The browns of the violin and the bow could be shaded lightly with browns of different depths. The man playing the piano could be a grey or black shade and the white shirt could be a creamier color than white. The woman to the left could be given a vibrant green color while the ribbons in their hair could be a rich, bright color that stands out against their hair color. The background could be light colors such as blues and greens to give the drawing a lighter, airy feel.