Thought I'd start a new thread on this, rather than post it under the Vega Gull one.
Here is a Herring Gull (blue CCAD) that I would have aged as a 2nd-cycle in the field, were it not for its colour-ring:
Except for the colour-ring, there is not much that betrays this bird's true age, I think. All primaries look really similar to 2nd-generation feathers: no white mirror, no white apical spots, slight brown tinge on inner primaries, only thin white tips to inner primaries. On the right wing, one or two inner secondaries show an adult-like pattern (blue-grey, with broad white tip), but such feathers are not present in the left wing, so, in theory, they could have been acquired through accidental loss. The only thing is probably the tail band, which is normally broader and more solid in 2nd winter.