Symbolism, Imagery, Allegory
When Anne's hair is compared to anything, it's to carrots. Not to a raven, nor a chestnut or even a tree trunk, but a common farm vegetable that grows in the dirt. No wonder she gets so mad. (For more on why Anne gets ticked off, check out the "Appearances" theme.)
Anne's hair's a very loud, very colorful marker of her difference from everyone else in Avonlea. We don't meet a second redhead anywhere in the story—Anne's the only one. Anne thinks her hair makes her ugly, but she forgets that because of its difference, it makes her striking. She's someone you can't ignore; she's someone you don't forget.
When she recites at the White Sands concert, her friends hear an artist say,
"Who is that girl on the platform with the splendid Titian hair? She has a face I should like to paint." (33.44)
"Titian" means "red" or "auburn" (after that painter Titian, who loved him some redheads). So though her hair has turned to a prettier auburn now that she's older, it's retains some of its redness. Older Anne still stands out in a crowd.