User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-10869825-20130905013302/@comment-7742465-20130905223128

It's pretty common for cover artists to change the color of a main characters hair or skin. It's just because that's the image of the character that's in their head. Take the Maximum Ride series for example. In the books, the main characters hair is described as brown with blonde streaks, but on the covers her hair varies from dark brown, to completely blonde, to white.