The Great Black-backed Gull has a dark grey to black back and wings, yellow bill and pink legs.
The similar looking but smaller Lesser Black-backed Gull is easily identified by its yellow legs.

As a rule, adult gulls attain their adult plumage and reach sexual maturity in two to five years, depending on the species.
Juvenile gulls do not look like their parents, but instead have a distinctive streaked brown plumage.
Over the years their plumage changes from brown to, usually, white or white-blue.
In general, males average larger than females and have slightly to distinctly bigger bills and larger heads.
The Great Black-backed Gull attains adult plumage at an average age of four to five years.

1st & 2nd photo: An adult. Sandy Point State Reservation, Plum Island, Ipswich, Massachusetts, USA, 20 September 2009
3rd photo: An adult together with a Laughing Gull. Cape Cod, Massachusetts, USA, 10 August 2017
4th photo: 2nd winter juveniles. Cape Cod, Massachusetts, USA, 10 August 2017