There are a small handful that are explicitly described as having a different hair colour, but there is a disproportionately large number of blonde characters.
The reason for this is that it's a cheap metaphor.
Blonde hair looks golden, so it implies both worth and morals (see: 'as good as gold').
So gold is today's theme, and sometimes bucking a trend can be an interesting experience.
Not as Good as
Eyes as blue as the Aegean and hair that shone like the gilded sun, Princess Roisin sat in the rose garden playing with the birds.
The king sighed as he leaned against the archway into the garden.
“Roisin!” he barked, making her jump, “let the birds go!”
She pouted and released the doves caught in her traps.
Her father strode over and picked up the one lying on the bench with its guts hanging out, breaking its neck with deft hands.
“The servants told me you were doing this again.”
“Which ones?” she asked brightly.
“Not after last time, Roisin.”