European herring gull
At least in the south-west part of the Baltic Sea and surrounding areas, the European herring gull ("L. argentatus") actually can be seen with yellow legs. They are not considered as a subspecies, since they regularly breed with grey/flesh-coloured legged herring gulls. The offspring may get yellow or normal-coloured legs. They must not be confused with the in general yellow-legged gull ("L. michahellis"), which are more common in the Mediterranean area, but single birds may reach more northern seas.