It's true that hybrids make ID of large gulls even more complicated than it already is. Sometimes it might be better to mark certain birds as 'showing characters of', rather than name them with certainty.
Both birds, however, look as identifiable.
The first one looks really pale, with hardly any streaks on the hind neck, the side of the breast, flanks and on its belly. It is always a good thing when there are other gulls on the same photograph, so you can compare colors. The upperparts look pretty pale as well.
This combination (very pale, almost white below and very pale on back/wings) strongly points to Mongolian Gull [Larus (vegae) mongolicus]. I'm not so sure of the age, because not much details are seen, although the bird seems to have an all dark bill, which would match 1st winter. Or it is a 2nd winter.
The second one is much darker and shows much more streaks on the head and the rest of its (visible) body. The upperparts show rather dark feathers (certainly if compared to the Black-headed Gulls). This would fit Heuglin's Gull [Larus (fuscus) heuglini] nicely, as other species such as Mongolian and Vega Gull have less dark upperparts. It looks much too slender to be a Slaty-backed Gull. I presume this bird to be in its second calendar year.
