Pass Card

In Fallout 3, s are items that are randomly dropped by ghouls or centaurs.

There is no place where a pass card is guaranteed to be found. A ghoul has a 1 in 80 chance of dropping a pass of either color; for a centaur the chance is about 1 in 170. The cards are accepted by Protectrons in lieu of metro tickets in specific areas.

Trivia
The presence of these items in Fallout 3 may be a nod to the preceding games in the series as Fallout and Fallout 2 featured Red, Blue, and Yellow Pass Keys which were used to open corresponding doors. There has also been a green pass card found and noted by some people

This may also be a DOOM reference. In DOOM player can find and pick up red, yellow and blue pass keys to open certain doors.