The difference between great and bad painting can in many cases be attributed to colour choices.
Brown hair, a green dress and red sleeves and may all appear together in Leonardo da Vinci's - Lady with an Ermine, but why did he choose that shade of brown, that shade of red and that particular shade of green?
Partly his decisions would have centered around trying to reproduce what he saw in front of hime, but partly it would have been about achieving a harmonious balance on the canvas.