They didn't have blue-grey mottling. It's probably a mistake or interpretation made by the book's artist coupled with the poor print process they used to make the book.
The photo must be kinda funky... The color is way off (old photos can do that) because it's flying over Dover in formation with another 110 (one taking the pic) and they would only have standard splinter camo on during the BOB. Only later and in different theaters did they seem to come up with various paint schemes (not counting night fighters)
EDIT: But we could do a standard splinter camo without mottling (as in this picture) to bring a little variation to what we have in-game. Wouldn't look much different, but it would probably be noticable.