Finding the "correct" color is usually finding the color that you prefer, not necessarily the color that is called for. You would think that all planes start out the same color when they come out of the paint shop, as Eric (@airbum) poiinted out, but I think there may be a little variation in the paint even then. I had a job just before I retired from the Air Force that allowed me to occasionally visit the aircraft paint shop. I was amazed at the variables introduced right from the start. You could get slight variations from the paint manufacturer, different paint manufacturers, variations in the amount of thinner put in, differences in how much paint is applied, and so on. All of these change the way the paint looks right from the paint shop, and then you throw in climate differences or seasonal differences, how much they were flown, if they were parked on a flight line or in a shelter, panel repair, etc and you get a bunch of different shades the plane could be. And the modeler often doesn't know any of this info. So, now you are depending on color info in a picture, which again, could have a thousand variables into how the color is shown. I think photos are never more than a guide.
I agree, Spiros (@fiveten), that the dark green with the red Danish roundels looks awesome. I was looking through my decals yesterday and found some decals for a Danish F-100 and F-84 that may need to move into the build list.