I kinda work on the unofficial WIAB canon that William, Leia, and Katie are siblings and Dylan and Helen are their parents. So, when I say the hazel/green/yellow is a Zane trait, it comes from Dylan's family line. Leia has Dylan's eye colour, Katie has Helen's, and William has a bit of both... I'm not saying that's how genetics work, 'just that it may be a plot device.
Off the cuff, I can no longer remember if Joe is related to Dylan, making Scarlet an auntie and Katerina a cousin. Joe does appear in the old WIAB, 'can't remember his eye colour.