How do I wire a door bell?
I don't think the use of these colours are standardised (outside of use in plain old-fashioned land-line telephone systems). 

I would disassemble the other end and see how the wires are connected up there. Most likely, two are connected to a push-to-make switch.

If you are sure the system is unpowered, you could use a multimeter to check all 6 combinations of two from four wires for DC voltage and, if no voltages present, check all 6 combinations for changes in continuity when the call-button is pressed.