Basic Trope: The hero's main love interest is a redhead.
Straight: Bob falls in love with Alice, who is red-haired.
Alice has two friends who are blonde and brunette respectively. They are both dateless, while Alice always has men chasing after her. The friends then dye their hair red and men immediately start chasing after them, too.
Downplayed: Alice's hair is a very dark auburn, and looks red or brown in different lights.
Bob's first girlfriend was red haired and their break up hurt Bob deeply, so he is trying to recreate his relationship with her. Alice happens to have the same hairstyle and features as her.
Or Bob has always been attracted to women with red hair because of how uncommon it is. He is the admired hero, so he gets who he wants.
There's a love triangle with Bob, Alice, and Charly (a blonde). Bob chooses Charly over Alice because the red hair was a turn-off for him.
Alternatively: The redhead wants the hero.
Or all villains want redheads.
Subverted: Alice thinks Bob only likes her for her hair colour and dyes her hair brown. Bob still fancies her.
Double Subverted: Bob then immediately cheats on brunette Alice with a random redhead off the street.
Parodied: Alice is introduced as a shy brunette who has no luck with men. She dyes her hair red and men immediately fawn over her and throw themselves at her feet. Men start fighting over such things as who gets to serve her tea and fan her while offering to carry her in a litter down the hallway.
Zig Zagged: ???
Bob and Alice are not romantically linked in the story because Bob wants blondes, just like everyone else.
Or Bob may like Alice, but does not like her specifically because of her red hair. If they break up, he's content to find a different girl, regardless of hair colour.
Enforced: The director has Author Appeal for redheads and insists that the love interest be red haired.
Lampshaded: While getting ready for her date with Bob, one of Alice's friends asks why she is so confident and Alice replies "You've seen the movies; the redhead always gets the guy".
Invoked: Bob has just had a nasty break up and his friend wants to help him take his mind off of his ex, so he brings in a hot redhead to make Bob forget about his "dowdy brown-haired ex" (the friend's words).
Exploited: Alice and Charly are vying over Bob and both of them are red-haired. Alice sneakily dyes Charly's hair brown while she's asleep in the hopes that this will make Bob want Alice more.
Discussed: Bob's friends are talking about what his future wife will look like. The first thing they say is "Well, obviously, she'll have red hair."
Conversed: The characters are watching some dating reality show with a blonde, brunette, and redhead as the three choices. When the presenters discuss which candidate the guy will pick, they say the redhead has clear advantage.
Deconstructed: Alice starts out as a blonde or brunette and tries to catch Bob's eye, but he isn't interested in her. She thinks he will fancy her if she dyes her hair red. He admits she looks better with red hair, but he doesn't like her because she is really a nasty person. He then calls her out on how shallow she must be if she thinks changing her hair colour would get him to like her, when she already had no problem with other men.
Reconstructed: After being told her personality is the reason Bob doesn't like her, Alice makes the effort to become nicer and less shallow. A few months pass and her natural brown hair grows through. After a while, she runs into Bob, who is flattered that she made the effort to change just to impress him. He asks her out and then casually comments, "You know, I liked the red hair."
Played For Laughs: Bob is shown as being unable to resist the charms of a redhead. A mugger who knows Bob displays posters and photographs of various red haired women in an alleyway and Bob is so transfixed that the mugger is able to steal his wallet and car keys without him noticing.
Played For Drama: Alice is a teenage girl and her new stepfather has a fetish for red hair. Over the weeks, he makes various comments about her red hair until, eventually, he makes a pass at her. Being young and naive, Alice accepts this and enters into an affair with him. Eventually, the stepfather uses her red hair as an excuse to rape her, saying that she's too fiery because of her hair and he has to calm her down.