I may be wrong, but isn't this the aircraft flown by "Ginger" Lacey, when he was instructing at Hawarden ?
If so, then I had an article years ago, somewhere, where he explained the colours, with a yellow nose, as I recall, to "rub home" the message of the "yellow-nosed b@stards" to his students, before they were posted to squadrons, when he'd dive down onto them ( the student pilot's flight) out of cloud or sun.
I think the tones are probably the effect of lighting, and the reproduction from the original print, with the yellow being fresh, possibly "washable" paint, hence it's brighter appearance. That said, i agree that it does give the appearance of white in this particular rendition of the photo.
The code letters are very probably Medium Sea Grey, and the underside "Sky", or a variation of one of the "mixes" used, with the "Sky" band either dirty underneath ( Hawarden is / was a grass field, in a rather wet area on the borders of England and North Wales ), or a combination of dirt, shadow and the printing / reproduction process.