User blog comment:Sim0ns11/Lucy's hair color/@comment-31036366-20171209000017/@comment-31753089-20171209121058

You're absolutely right. Plus, the blondes are the more feminine and interested in fashion and appearance, minus Lana (since Lily's favourite color is lavender, I guess she will be feminine). The brunettes are the less feminine, and Lucy could be the exception: when Luan is dressed up as Lucy in  "Cover Girls", she has black eye make up on, meaning that Lucy usually wears make up, making her the feminine brunette.

Plus, in the opening theme song, look at how the sisters are arranged:

https://vignette.wikia.nocookie.net/theloudhouse/images/4/4b/1_Boy_and_10_Girls.jpg/revision/latest/scale-to-width-down/320?cb=20160912075453

There are two blondes at the top (Lori and Leni) and two blondes at the bottom (Lola and Lana), then there would be two brunettes at the left, if you include Lucy (with Luna), and two brunettes at the right (Luan and Lynn), and finally a blonde at the left (Lily) and a brunette at the right (Lisa). This could be a clue on Lucy being a brunette. What do you think?