Here are a few gulls in their second plumage cycle (often referred to as their second year, but that is not always the case). The first two birds are four-cycle gulls. On a four-cycle gull, the second cycle looks much like the first, but the mantle feathers are coming in gray.
Western Gull, second cycle. Note the blackish primaries and tail, the dark gray mantle, the heavy bill, and the fairly extensive mottling on the underparts.
Herring Gull, second cycle. Note the pale iris and the slender pinkish bill with the black tip.
Three-cycle gulls skip the all-brown phase seen on young four-cycle gulls. (So a first cycle Ring-billed Gull has a gray mantle like a second cycle Herring Gull.) The second cycle on a three-cycle gull shows a gray mantle and wing coverts, but the primaries lack the white tips and mirrors seen on an adult.
Ring-billed Gull, second cycle. Note the sloping forehead, pale eye, yellow bill with dark near the tip, fine streaking on face, and crisp scaly pattern on breast and sides.
Mew gull, second cycle. Note round head and very thin bill.