You know, I think that if you are happy with your build, which looks really good from where I am sitting, then I would just leave it alone. It looks good. Who cares if it is historically accurate?
I just completed my set of Airacobras. I used Bert Kinzey's Detail and Scale book as a reference. Yes, I did not see any examples of 'Cobras with blue undersides but the book is not exhaustive. Who knows, maybe there was a blue underside 'Cobra roaming the skies of the South Pacific. Another thing I notice is that you painted the landing gear legs aluminum or silver. Back to the book, I do not see any examples of that. Everything I see is the gear legs are painted "Bell green", kinda a dark green, not really green zinc chromate. But again, if you are happy with your work, and you should be, then I would leave it alone.