Answer = When it is a Herring Gull with yellow legs! I don't know if this is a Guernsey phenomenon, but when I began my gull studies here in 2008, I was regularly confused by apparent "Herring Gulls" which have yellow legs (even bright yellow). This is most obvious in the summer, when leg colouration intensifies with the breeding season. I've always been cautious to identify true Yellow legged Gulls (i.e. Larus michahellis) because of the presence of these yellow-legged argentatus. However, I've now seen several michahellis in the hand and also some of the peculiar argentatus. One such bird in White 6.UU3 (originally ringed in May 2010 in our garden, and now seen c 20 times in Guernsey including today at Chouet Landfill). The photos below show just how yellow the legs can be...but note the normal pale mantle of argentatus and also the orange (not red) orbital ring.
I do not know if these yellow-legged pale mantled birds are hybrids, and if so what has crossed with argentatus, - presumably mostly fuscus, but perhaps michahellis?
Adult Yellow-legged Gulls tend to stand out in Guernsey because they are always seen in the presence of Herring Gulls. The mantle is so dark that it is almost always the factor which draws attention to the gull. Then you notice the (normally) bright yellow legs and the red orbital ring. Often it is also possible to see the larger red spot on the bill. 2nd and 3rd Winter michahellis are also fairly easy to identify in Guernsey (showing most of the above distinguishing marks)...but we are not making a very good job of identifying juvenile and 1st winter michahellis. They must be scarce, but present in small numbers every year,...but it it remains a gaping hole in our ornithological knowledge on Guernsey at the moment. Sooner or later someone is going to master their identification here!
All above photos are Herring Gull (Larus argentatus)White 6.UU3
Below all Yellow-legged Gulls (Larus michahellis)
Herring Gull on left Yellow-legged Gull on right (note darker mantle)