I didn't see any juvenile California gulls. The next several images are second summer birds. Some of them, such as the first bird, would be unlikely to draw the eye in Michigan, especially if they were alone. For the most part though, the young California gulls had a little more brown through the eye, a duller bill, and a longer more tubular bill than a Herring gull would. Most of them look a little smaller headed to me as well.
The next two images are of the California gull I had at Tiscornia in very late May a few years ago. The benefits of studying lots of the birds in places they're supposed to be becomes clear given that at the time I aged (and submitted) it as a bird coming into it's second summer. It's 3rd summer. It's also probably a different subspecies. I can't remember which subspecies has a slightly darker mantle. I think the differences between the apparent mantle shade in these 2 pics as compared to the last ones on this page are due to lighting and camera exposure.
And finally, some adult type birds. Again, they seem to have a little more black through the eye than a Herring gull would. The bill is again longer and with less of a gonydeal angle than a Herring gull's for the most part. The dark eye is also worth noting though I have seen adult Herring gulls with at least one dark eye.
Ignore the primary length of the first bird, it's moulting the outer primaries.