On thinking back to Eva asking him about Corrine and the brunette thing earlier in the book or maybe at the NC beach house, I cann't remember exactly where the conversation took place, I think Gideon liked Brunette's because Corrine was his first true girlfriend and at the time I believe the thought he was in love with her, he proposed to her so maybe he thought it was love and with her being the first girl who had really shown affection to him he fixated on the brunette thing from her.
Â
Like he said to Eva  he didn't think he was making a preference, comparing everyone to Corrine or the brunette thing and it was not intentional which makes me thing subconciously because Corrine was brunette he related, warmth, friendship, affection with that colour.Â
Â
just a thought:-