Gulls take 2-4 years to mature. The plumage during the juvenile and subadult years differs from the adult breeding plumage, and the plumage also differs between summer and winter. Thus, there can be 7 or so different plumages for each species of gull, making identification difficult. Presented here are a collection of photo of young gulls. Gull experts could sort them out, but it isn't worth it to me anymore.

Young gulls tend to be dark overall, including the bill.