For reference: images of (presumed) hybrid Herring Gull x Lesser Black-backed Gull ringed in 2021 as Green YDJB as part of our project in the IJmuiden Forteiland colony, the Netherlands, and seen in subsequent years.
The grey mantle is darker than that of our breeding European Herring Gulls (Larus argentatus argenteus), and paler than our Lesser Black-backed Gulls (Larus fuscus graellsii, Dutch intergrade).
The mantle is also slightly darker than our palest (presumed) hybrid Lesser Black-backed Gull x Yellow-legged Gull YAHH, and paler than our typical hybrids (such as YBVA and YBAM).
Primary P10 has a large mirror with a subterminal band (broken in 2023), P9 has a mirror on both webs with a subterminal band. P5 has a broad black band.
None of our adult (presumed) hybrids Lesser Black-backed Gull x Yellow-legged Gull have a mirror on P9.
A black band on P5 is also seen on some of the Herring Gulls in our colony (such as on YBUF), although not as broad on the inner web as seen on YDJB.
The orbital ring is orange and not yellow as in Herring, but in range for Lesser Black-backed.
The legs are pale yellow but were deeper yellow during the 2022 breeding season. Interestingly, when we caught it on a nest in 2021, the legs were as pink as the legs of a Herring Gull.
The images below were all taken by me in the IJmuiden Forteiland colony.