Silver Gulls with black sub-terminal band across tail feathers

When in Broome, Western Australia in 2002 and again in 2015 I saw Silver Gulls with all the features of juvenile birds but which have a distinctly black sub-terminal band across the upper surface of the tail. The juvenile Silver Gulls I see around Nambucca Heads may have a light brownish band but not black. Checking the literature (HANZAB, Pizzey & Knight, Simpson & Day, Morecombe, Slater) all mention ‘brownish’ except the Slaters who say ‘dark’. None describes black. Am I mistaking a dark band for black or am I observing an undescribed Broome or Western Australian feature of the Silver Gull or perhaps I am mistaken when I identify them as Silver Gulls. Can someone sort me out please. Evan Cleland.