Andrew- Your question makes some assumptions. There are no observers moving at the speed of light with respect to other observers. All observers must move slower than the speed of light with respect to each other. There are no "stationary" observers, unless you wish to call all observers stationary. Each is stationary in its own reference frame.
What you can say is what the Doppler shift multiplier of the light frequency is as an emitter approaches you at speed v:
((c+v)/c-v))
1/2or
departs from you at speed v:
((c-v)/c+v))
1/2.
Mike W.
(published on 02/20/2013)