I’m pretty certain she bleaches her hair  just to lighten it a bit. If you look at older photos you can see how her hair is darker underneath and then it get’s lighter again.

Her part also tends to have more dark hair depending on the timeline. Ross is a natural dark blonde but Rocky and Ryland have naturally brown/dark blonde hair.  So my best guess is that she has a hair color like them but bleaches the top layer to keep it super blonde. 

And thank you! <3