Herring Gull with yellow(ish) legs at Alnwickhill

A very quick visit to Alnwickhill Water Treatment Works this evening unexpectedly produced a Herring Gull with yellow(ish) legs. Clearly not a Yellow-legged Gull based on structure, pale grey upper and black wingtip only extending to P6 rather than P5 or further. Yellow-legged Herring Gulls make up at least 50% of some populations in the Baltic, but they tend to be much less common among local argenteus Herring Gulls. It has been suggested that some dietary component can cause this brighter leg colour. In the first shot, can you guess which one it is?...