Gaivota-de-Patas-Amarelas (Juvenil)

Thanks for stepping in with a fuller explanation Pedro. I am still working from old books but perhaps this one would be worthwhile for anyone who is really interested in the subject: http://www.amazon.co.uk/Gulls-Europe-Ame...

Yes, large gulls such as this one have 4-6 types of plummage.
Juv/1st Winter, 2nd Winter, 3rd Winter, 4th Winter/5th Cy (calender year).

So it basically takes them 4 years to reach adult plummage, and in the meanwhile you get several transitional plummages.
The "toughest" ones are the earlier ones. I can't distinguish 1st Winters from different species, for instance (Larus michahellis, L. fuscus, L. argentatus, L. cachinnans...), they are too brown-ish and have very similar features. But as they grow older it gets easier to distinguish them. In Portugal, the only possible confusion with juveniles is between Yellow-legged and Black-backed gull, as all the others are quite rare and it takes a lot of luck to find them.

During Winter, Black-backed gulls (Larus fuscus) are the most common, during Summer, Yellow-legged gulls take their place. I think the latter ones are the only bredding species here, there are several colonies including in Berlengas, Sagres, etc.

I hope I've given you some context and answered your question.

Now, for the full id of the birds (considering I'm anything but a gulls expert):
I think all of them are 3rd cy (equivalent to 2nd Winter) yellow-legged gulls, as Malcom stated.

Finally, I'd like to say there are other species of gulls (mainly smaller) in our country, but these are quite different and are easily distinguished.

Thank you soo much Malcolm. Can you explain me the differences between the several types of plumage? Is it related with the age of the bird?

Many Gull species have white spots on the wing tips and these are variable between the species so you need to study the variations in detail to be able to detect the small differences.