The photo clearly dates to just after a change of markings was ordered in May 1942. Note that the ring sizes on the roundels of aircraft 2 & 4 are different from the other two as are the tail markings. 2 & 4 represent the proper C1 roundel introduced at that time. The other pair have been repainted, at least partially, to comply with the new design. The changeover took some time to implement in full.
As for the code letters, prior to this change grey was the standard. After it was initially a colour "appropriate" to the Command. According to orders issued in Oct 1942 Bomber Command codes officially became red. The Bostons of 88 squadron belonged to 2 Group Bomber Command.