In Bradbury's Fahrenheit 451, the dandelion does fail to rub off on Montag's chin, indicating, according to Clarisse, that he is not in love. In addition to this leading to Montag's understanding of how much his marriage is lacking, it helps lead to his understanding of how much his entire existence is lacking.
This dandelion incident is part of the entire package that Montag is exposed to by Clarisse. The fact that she even thinks about "love" is new to Montag. Also, her family actually spends time talking. She contemplates nature. Most of all, perhaps, she spends time thinking.
All of this, including the dandelion incident, plays a part in Montag's "seduction," if you will.
Of all things, she rubs the yellow flower from a dandelion under her chin. She says that if she does that, and the color rubs off on her skin, it will mean that she is in love. When she rubs it under her chin, the color does come off (which surprises me because you think with her being such a rebel she'd have trouble finding love).
She then has Montag try. It does not come off on him. This is part of the process that makes him sad about the way his and Millie's life together has ended up.
In the world of Guy Montag, the protagonist of Fahrenheit 451, passion is pretty much nonexistent, a luxury the ruling regime can ill-afford. For Montag, the stability of his daily routine is comforting, and his relationship with his wife, Mildred, is sterile and devoid of emotional sustenance. Mildred does little other than gaze at the television, although a suicide attempt clearly indicates inner turmoil to which Montag may not be privy. When, however, early in Part I of Fahrenheit 451, Guy encounters Clarisse McClellan, the world as he has known it changes.
Bradbury's society is ruled by an autocratic regime that seeks to control not just movements but thoughts, and because books represent thought, they are considered a threat. The individuals who populate this world have, with some notable exceptions, conformed to the regime's dictates, and Mildred, at least on the surface, represents one such example. That is why the encounter with Clarisse, a vibrant, cheerful 17-year-old teenager, is so initially disconcerting. Clarisse seems not to have gotten the proverbial memo about personal conduct, noting on this initial encounter that she is "crazy" and likes "to smell things and look at things, and sometimes stay up all night, walking, and watch the sun rise." Clarisse, in short, is unlike anybody Montag has ever met, and she becomes a symbol of the liberating potential of rebellion against this repressive system.
Bradbury uses Clarisse to emphasize the passionless life that Montag has been leading, and it is in this context that the experiment with a dandelion enters the picture. Observing these weeds with their bright yellow flower, Clarisse asks Montag whether he has "ever heard of rubbing it under your chin," at which point she proceeds to do precisely that, leaving yellow traces under her chin. When she suggests that Montag try it, he complies only to discover the absence of any yellow residue under his chin, indicating that he is not in love, which prompts the following exchange:
"What a shame," she said. "You're not in love with anyone."
"Yes, I am!"
"It doesn't show."
"I am very much in love!"
Montag is insulted by the suggestion, on the basis of this silly experiment, that he does not love his wife. Clarisse's little game, however, proves prescient as Guy and Mildred's relationship deteriorates.
In conclusion, and to summarize, it is a dandelion that Clarisse rubs under her chin.