I always think of LBBG as distinguished from almost all other large gulls (regardless of what point they are at on the dark/light scale) by having a distinctly brownish-grey, as opposed to bluish-grey, tint to its mantle. This bird looks bluish enough for me to get an instant impression of "not graellsi"
The reality is a little more complex than that in my experience.
Graellsii, especially birds from the west of their range (like here in the UK) are known to be bluer in hue, and obviously lighter in tone than
intermedius. Darker still, and reportedly tending towards the reddish/purple end of the spectrum are
fuscus, though I have no direct experience of the latter.
Attached are 4 images of adult LBBGulls from northern England.
The first two illustrate birds that I think of as typical of British-breedng
graellsii, with an obvious slate blue-tone. They were taken with a phone camera, so apologies for the poor image quality. The phone's white-balance settings might also be responsible for altering the colours a little, but I still feel that they are mostly representative of typical British
graellsii.
The third image nicely compares the relatively cold, bluish tones of Black-headed Gulls (which to my eyes are very similar in colour and tone to our local
argenteus Herring Gulls); a single sub-adult Yellow-legged Gull (a couple of shades darker than argenteus and distinctly warmer in hue) and several Lesser Black-backed Gulls. I hesitate to label these as typical
graellsii as I've often thought that many of the birds that pass through northern England in late summer/autumn are different in tone to our local breeding LBBGs. They are both darker in tone and warmer in hue, and I've often suspected that they might originate from the so-called 'Dutch Integrades' that are a mixed 'hybrid population' of
graellsii x
intermedius. Either that or they represent a different clinal variation within normal
graellsii? Unfortunately I don't have enough ringing recovery data to prove this either way at the moment, though a few that I have traced so far have proved to be from Dutch colonies.
The last image compares what I think is a fairly typical blue-toned
graellsii and a bird which is both much darker and warmer in colour (probably
intermedius or perhaps even
fuscus?). I think the warm evening light is playing a part here, yet both are still clearly different in colour, regardless of how dark they are.