Blondes might have more fun, but Ani wants to keep her golden locks under wraps in this novel. Why? They could give her royal secret away. When she gets to Bayern, we get the dish on the hair in each country:
Ani jumped and stared at the noise and colors, a sea of hatted and scarf-wrapped heads, faces marked with dark brows and lashes, soldiers bearing iron-tipped javelins and brightly painted shields. One among them had fair hair. (6.52)
Um, okay—so in this novel, hair color is a way to tell where someone's from. It's pretty simplistic and a little strange that what people physically look like—instead of, say, something they wear—is an indicator of where they're from in this book, but so it goes. Ani's blonde hair is a dead giveaway that she is not only not from Bayern, but also most definitely from Kildenree.
So of course Ani doesn't want people to discover her golden hair—word could get back to Ungolad and his men that she's hiding out in the palace pastures, and that would be a pretty unfortunate development for our main girl. So to keep this from happening, Ani hides her hair, just like she hides her true identity.
Pretty soon though, Ani's locks make an appearance to help her convince the other forest workers that she is really the princess. Seeing her yellow hair, they understand that she is most certainly not from Bayern, which lends her much-needed credibility as she tries to rally the troops to help her oust Selia.
And because of this, ultimately Ani's yellow hair does more than just symbolize who she truly is and where she's from—as she goes from showing it to hiding it to showing it again, her hair represents her journey to self-acceptance and stepping into the powerful role she was born to occupy.