I know they were ordered by the French and given French serials. That I don't dispute. But, the aircraft as depicted in the photo were painted exclusively for the RAF, of that I have no doubt. If it ever was painted for the French, it's been painted over, including any national markings. On the DB-7s built for the French, Douglas painted no fuselage roundels, no camouflage, and never tail stripes on the vertical stabilizer--always on the rudder with aircraft type and serial painted over it in black.
In 1940, the world of black and white film was still in transition. Many types of black and white film were orthochromatic, the standard of the 1920s and earlier. By the 1940s, most film was panchromatic, but not all. We're used to panchromatic black and white photography in which blue is darker than red, but orthochromatic film, common in the 1930s and still around in the 1940s, is the opposite, with red being darker.
An old Kodak advertisement for "new" (1930) panchromatic movie film helps to illustrate this better.
ORTHOCHROMATIC ----------<>---------- PANCHROMATICTake a look at two Hawker aircraft photographs in British service, the first from around 1935, the second from late 1941.
See? The roundel colors (blue, white, and red from the outside to the inside) are, in real life, very similar, but in this photograph they are very, very different shades of gray.
The actual shade of blue may be off, but I'm sure it goes Red->White->Blue from front to back. I think it's just the film or lighting. See these scans from Squadron/Signal's "A-20 Havoc in Action", pages 7 & 8.
I apologize for the poor quality of the scan, but you can still see the darker red in the fin flash on all three pictures of factory fresh DB 7As. It's even darker in actual book if you have it. Maybe it was orthochromatic film instead of panchromatic? You can also see the same painted-over RAF roundel and exact same camouflage patterns as the aircraft in the above photo.
Cheers,
Logan