I do get this question quite often from law enforcement, especially when a department is trying to decide whether to let each officer choose a preferred color combo or find one specific color combo that would work for most.  Our Advantage Tactical Sight system was designed around the concept that most, especially men, saw colors differently. 
With that in mind, so the concept goes, one should choose a color combo most visible to him or her in variety of lighting.  However,  there is one color combination that seems  to work for many users.
For tactical considerations, many, including me, see red rear with white front very well in a variety of lighting situations.  This combination is quite visible in lower lighting environments, yet gives a clear sight picture in normal lighting.  The only problem I’ve had with this combo is in action shooting competitions.  When shooting at freshly painted white plates on a sunny day, where the plates are at 15 yards and beyond, our white front sight insert could blend with the white of the plate.  When the plate is closer or the day is cloudy, this is not a problem.  However, for a non-firefly color combo, I believe this is one of the best tactical color combination considerations, as long as your target does not look like a white plate on a bright sun shiny day.  The reason why I believe so strongly in this color combo is because almost everyone can see it well, regardless of individual color preference. 
For a good Firefly color combo, there are several that seem popular: 
            - Yellow Firefly with a red rear sight insert; 
            - Yellow Firefly with an orange rear sight insert;
            - Green Firefly with an orange rear sight insert
            - Green Firefly with a green rear sight insert,

That last one really works well for those who prefer a mono-chrome presentation.  Once you acclimate to seeing a solid colored pyramid, it’s very fast and precise.  Of the monochrome, the green\ green is the most popular.  Also, white/white and, to some degree, yellow/yellow seems to work well.