Images 1-4, bird A: overall pale and very cold-toned, all greyish-browns and frosty off-white (further accentuated in this image by the contra lighting and the high ISO and colour noise reduction settings, all of which tend to desaturate the image slightly); the only green/yellow tones are on the edges of the remiges/rectrices and a hint at the alula; the pallor of the crown emphasizes the short dark lateral forecrown stripe which extends to just behind the eye; paleness on the bare parts is restricted to the cutting edges of the upper mandible and the feet.
Images 5-6, birds A and B (bird B on the right in image 5, left in image 6): bird B showed slightly more warm ochrous/ochrous-buff tones on the ear-coverts and breast sides than bird A but was still a strikingly greyish-brown individual; both birds showed obvious wingbars due to pale-tipped greater coverts. One of these birds uttered a monosyllabic tristis-type 'peep' twice in quick succession during my initial sighting, however, none of the Chiffchaffs were calling with any regularity, and neither were heard to call subsequently despite observations spanning two and a half blustery hours.
Image 7, bird B: taken from the opposite side of the sewage works compound, i.e. 'with the light', and at a slightly lower ISO setting, note the difference in saturation/plumage tones as compared to images 5-6; this image probably shows the 'truest' impression of bird B's plumage, with bird A being slightly greyer/colder again. Having only heard one of the two birds call, I would only claim one as tristis but, were I local, I'd keep going back until I'd nailed the calls of both,... 'cos I'd be surprised if the second bird didn't go 'peep' too.
Images 8-10: three increasingly yellow/green toned individuals (there were several others knocking around); the bird in image 8 shows a greyer crown and nape, a reasonably obvious lateral forecrown stripe, and ochre washed ear coverts; whilst the bird in image 10 had lots of yellow in the supercilium and an extensive pale lower mandible. Wouldn't it be nice to pop the bird in image 7 in the abietinus box and 9-10 in the collybita pigeonhole, everything would be so neat and tidy then.