Yellow is probably the common, pink are the four windings. Use a multimeter, the resistance between two pink wires should be double the resistance between a pink and the yellow wire if this is the correct wiring. Or, take a battery, connect yellow to one end of battery, then take turns connecting each pink wire to the other end of the battery one at a time. Stepper motor - Wikipedia, the free encyclopedia
As Norlin states, the yellow wire is most-likely to be the common. The pink wires are most-likely to be the phase winding wires.
Have a look at Nigel's Unipolar Stepper Tutorial here:
Although a six-wire Unipolar Stepper Motor is depicted in Nigel's tutorial, a five wire Unipolar Stepper Motor also applies since the two + common wires are joined together inside the motor and not on the board.