The tickets, explained
Tickets in The Freak Circus are not collectibles. They are an access system: they decide which tent you can enter and which category the circus files you under. One of them decides your ending outright.
Official sources last checked
The one-line version
If you are still holding a pink ticket, you are walking into the game's only bad ending. Help Pierrot and he takes it off you.
Tickets that appear in the game

Gold — general admission
In gameGiven by: Bought normally
Basic admission to the circus. Gold border, a black panel reading THE FREAK CIRCUS OF HORRORS, and a gold tent icon.

Red ticket
In gameGiven by: Pierrot, on Day 2
Admission. The Ticket Taker says it means front-row seats in the red tent — that claim does not hold up, and it is one of the small lies the game lets you catch.

Pink ticket
In gameGiven by: The Ticket Taker, as the "unknown man" on Day 1
The dangerous one. It grants access to the pink tent and marks humans so that they become either Fools or food once they go in. Pierrot takes it off you as fast as he can if you help him early on.

Green ticket
In gameGiven by: Harlequin
Admission with "special perks" that the game never specifies. Anyone telling you exactly what they are is guessing.
Black, cyan, purple and blue tickets
UnconfirmedGiven by: Never handed out on screen
The colour scheme implies one ticket per tent, which would give the Doctor a cyan, the Jester a purple and the Ticket Taker a blue. None of them is ever seen or given in the current build, and the black one has no known use at all.
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