Silhouettes (the shape a character makes by their overall shape and outline) are the most important design element for gameplay clarity.
In the thick of a frantic battle there isn't time for the brain to process which champion is which by voice or color palette or clothing. The quickest way for the eye to determine which character is which is by their general shape. That's why TF has his big hat, why Nautlus has his anchor slung over his shoulder, and that's why Jinx has thin whip-like ponytails while Sona has long flowing ones.
Even if they used each other's color palettes, Jinx running around with Sona's flowing ponytails Sona running around with Jin'x long thin braids would cause a lot of confusion in the thick of battle. You'd have everyone dumping their kill power onto the support, not by baiting or outplaying or out-strategizing the enemy team, but because the critical design elements that visually separate them have been flip-flopped.