I think that just comes down to the "modelers" interpretation and not the actual color. In most cases as in with Model Master enamel paints it is spot on to FS colors. Now switch to an acrylic paint and the tone will be a hair off due to the chemicals used are not the same but are to be made close. Take in a different company who cannot get their hands on the exact pigment for what ever reason. Now we run into the big issue. Everything in modelling is an illusion, we are trying to trick the eye into seeing something that it reay is not. Now be it panel lines, weathering, and in this case paint colors. Perfect color that on a model does not look right what so ever is gunship grey, it "appears" darker on a model than it does on the real aircraft. Just like white "appears" too bright on a model than on the real aircraft. Pre-shading is un-realistic on real aircraft but we do it on models, but why? To fool our eyes to think we are looking at the real thing.
That particular model you are refferencing is that of the modelers interpretation on what they thought looked right. But here lies two issues, one it is not 100% accurate and it was based off a black and white photo with refference to FS colors. But here is the nice part, its a model! No one, and I mean no one will EVER be 100% correct because of scale difference which effects color tones. Colors also look different inside than outside, under natural sun light which these birds where exposed to (which is its own issue with fading) compared to tube or bulb lighting.
But to my point, you will have to paint it to what YOU see fit, because there will always be someone who said you did it wrong, but I would argue that it is in the eye of the beholder. You are making a kick arse Avenger! Keep at it chap!