Thanks for everyone's input into this, I'm still open to changing the colour if better info is found. The written reference to medium blue reads more like a vague description than a specific colour. It could be true blue/light blue or sea green, which is actually more of a blue/green colour. The steel planked runway on Oboe's colour shifted photo looks too purple to my eyes and like Devil says, the grass could easily be brown in patches.
I don't trust profiles/models/paintings for things like this. I've just seen too many examples over the years where each artist has used previous artists' work as their only reference and so transferred their mistakes over too. Thanks for ordering those books Lyric1, with any luck there will be a better colour photo in one of them.