Color scheme is mostly a matter of marketing. On a wing, the only people that get to what it would be the passenger: those who already paid for their tkt. Besides, as awareness and brand recognition is also important after the purchase, the cabin is full of branding that remember the consumers the airline they are flying by. So there is no point in using the wings to this task: paint adds weight and costs. There are better ways for marketing the airline during flight. At the airport, nobody actually gets to see the wing.
One of the wing tail painted that I really like:
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Photo © Fabrice Sanchez - Brussels Aviation Photography
in order: 721,146,732,763,722,343,733,320,772,319,752,321,88,83,744,332,100,738, 333, 318, 77W, 78, 773, 380, 73G, 788, 789, 346