Some early observations on colour power level on my part:
I might be biased by the fact that I haven't had a single Sealed pool where I didn't play Blue. But to me, it seems Blue is the best colour in the format due to its depth in above average commons.
Cloudkin Seer is probably the second best common after Murder,
Frost Lynx and
Boreal Elemental are solid top 10 material,
Octoprophet and
Winged Words are great cards you'll probably as many of as you can get, the first
Unsummon is amazing, and even its 2-drops are among the best in the format (
Moat Piranhas blocks every grounded 2-drop and almost every grounded 3-drop,
Metropolis Sprite can trade earlygame or pose 2 power lategame. The one 2-drop that I'd consider better is
Leafkin Druid). No other colour has this much depth.
The second best colour is between Green and Black for me, and IMO Black edges out slightly; mostly by virtue of having access to
Murder. There's some other quite good cards at common (
Agonizing Syphon,
Audacious Thief,
Boneclad Necromancer,
Gorging Vulture,
Bone Splinters,
Fathom Fleet Cutthroat). My main gripe with Black is the lack of impactful early drops;
Sanitarium Skeleton and
Blood Burglar are fine, but quite a step down from Blue's 2's;
Sorcerer of the Fang seems rather mediocre.
I've pegged down Green as the third best colour; and in a format where ruling the skies means ruling the board this may seem puzzling. However, Green has all the tools it needs to deal with opposing fliers in
Rabid Bite as well as two reasonably sized Reach creatures at common in
Netcaster Spider (trading for
Boreal Elemental is huge) and
Mammoth Spider. If all of this (and the Reach creatures at Uncommon/Mythic) fails, there's still
Plummet, too.
Thanks to this and the fact that Green creatures are fairly beefy by nature, Green is probably the best defensive colour in the set; and thanks to the likes of
Silverback Shaman and
Brightwood Tracker it can keep up the card advantage game.
Its biggest problem is turning on the offense since it has to slug through a probably fairly clogged board; if a different colour can take these duties Green is amazing.
Red and White are the worst colours. Red is mostly a worse Black, with quite a bit of solid removal and some ways to go late; but with very few fliers and not as good of a defensive stance. White wants to go wide, but the small creatures get blanked by about every blocker. Also both of these colours, despite wanting to be aggressive, have too few good 2-drops to do so.