Snow, initially, reads as a Pair archetype in GU with a very small bleed into W and B. GU very clearly stands out as the color pair with, by far, the most support for snow. However, all five snow lands can appear in boosters, since it would be unpractical to only reprint Snow-Covered Forest and Snow-Covered Island. To amass a critical number of snow permanents, it's therefor likely the GU player will pick up off-color snow basics. Arcum's Astrolabe is as much a tool to turn splash colors into the main colors as it is the other way around. The cute thing is that GU is traditionally the color pair that's best at splashing cards though, because green has mana fixing (e.g.
Rampant Growth), and blue is the best at card drawing, which also helps you find both your splash cards and the mana to play those cards. In that sense, Dead of Winter and On Thin Ice aren't so much an indication that the snow archetype runs in other colors, they're WotC capitalizing on the overlap between WotC's planted archetype for GU (i.e. snow) and GU's natural affinity with being the core of multicolor decks.
Does that mean snow is indeed of the Pair archetype variety then? No, because I actually think there has been an archetype missing from japahn's musings so far, which is probably what threw you for a loop. I guess you could aptly brand this archetype as the Pivot Pair archetype, as it shares qualities of both the Pair and the Pivot archetypes. The Pivot Pair archetype resembles the Pair archetype, in that the core of such an archetype very clearly spans across two colors. This is exactly what we see with snow, which unmistakably has a GU core, which means a snow player will end up in those two colors a great majority of the time. However, thanks to off-color snow basics, Arcum's Astrolabe being an excellent mana fixer at common (that won't get picked up by non-snow drafters!), and two excellent rare payoffs in other colors, the snow drafter will very often end up splashing cards, and this is where the snow mechanic starts to resemble the Pivot archetype.
The great thing about the Pivot Pair archetype, which GU Snow is in my opinion, is that it tackles both the "drafting on rails" downside from traditional Pair archetypes, as well as the "lack of cohesion" downside from the traditional Pivot archetype. The Pivot Pair archetype's biggest downside, in my mind, is that it's hard to pull off successfully, since it so easily slips into a Triangle, Tetra or even Pentagram archetype if too much support is put in one or more non-core colors (essentially turning them into core colors), or by not offering enough tools to successfully enable a drafter to venture into non-core colors. GU snow wouldn't have been as successful at being a Pivot Pair archetype if they hadn't printed the Astrolabe, for example. Another downside is that drafters unfamiliar with the format might conclude that an archetype is supported outside of the core color pair, and end up forcing the wrong pair, ending up with a sub par deck.
Anyway, I'll leave it up to japahn to do a full write up for the archetype