@Puchotek - maybe it's because "blue/green*" - 青 (ao) is the first character in "youth" - 青春 (seishun, or in a more literal reading ao-haru)
and there's nothing that reeks of youth like healthy, vibrant, sweat-in-the-sun sports like football (soccer for the yanks) 😛
* it's complicated 😅...