All five characters who get on the elevator have the color red in common: the Mechanic's satchel, the Old Woman's hair, the Young Woman's nails, the insignia on the Guard's uniform and the Salesman's tie are all red.
Number 666 references: the building number is 333 (666/2), the elevator number is 6, and stops on floors 23 to 42 (2*3=6, 4+2=6), When the hockey match on TV turns on right after the elevator stops, the time of the match is 1:42, (4+2=6), the floor that Sara wants to go to is 42 (4+2=6), the floor that Ben needs to go to is 39 (9-3=6).
M. Night Shyamalan admitted that Agatha Christie was an influence on the film, especially Christie's 1939 novel And Then There Were None. They both share the same plot of a group of people with guilty pasts trapped in an isolated location and die off one by one. The final plot twist is also the same. The villain is among them, thought to have been killed off earlier in the story.