In the topsy-turvy world of Fahrenheit 451, where firemen start fires instead of putting them out, kerosene comes out of fire-hoses instead of water. As a colorless liquid, kerosene is an ideal symbol for the emotionless, matter-of-fact way that the firemen go about their business. It's also hard to wash off. This represents the difficulty that Montag will have in completely erasing his identity as a state functionary. It's almost as if kerosene has entered his soul, his very being.
Kerosene's indelible properties also suggest the huge challenge that Montag, Faber, and other dissidents face in overthrowing this totalitarian regime. For even if they do manage to succeed in their rebellion, the lasting effects of the regime's tyranny will, like the smell of kerosene, linger on for quite some time.