Above shows some of the limited range in upperparts colors, compared to Brown-headed Gull (Larus ?)  Right-hand bird is typical of more advanced moult birds, typically paler mantle and with brighter yellow legs and largely unmarked head. Left-hand bird is also of the "paler" variety.  The bird immediately next to it, actually a sub-adult, is at the darker end of the birds present.

None of the adults present had completed their primary moult.  Although not uniform, most of the paler birds were more advanced, having dropped their outer primaries.  The darker birds tended to have their old outer primaries, along with more streaked and marked heads and legs that were not as bright (pea-green, fleshy or dull yellow etc.).  However, this leg color was shown by some of the paler, more advanced birds.