There she was, a beguiling light green, sitting on white tile. I found it quickly in Dominique Collet's Insects of south-central Alaska though the antenna feathering in the book's photo is much thicker than in my photo.
It's a Pale Beauty or Campaea Perlata.
Bug Guide tells us it can range from pale green to grayish-white, fading to yellow. They are found from
"Alaska across Canada to Nova Scotia; south to central California, Arizona and Colorado; in the eastern U. S. south to North Carolina.(2)"
Their habitat is:
"coniferous, mixed, and deciduous forests and shrubby areas; adults are nocturnal and come to light, but in the arctic where summer nights are short or absent, adults fly during the day"
This one had come in to the light of the bathroom when it was as close to dark as it gets outside this time of year.
"[L]arvae have been reported to feed on leaves of 65 species of coniferous and deciduous trees and shrubs, including alder, ash, basswood, beech, birch, blueberry, Buffaloberry (Shepherdia canadensis), cherry, fir, elm, hemlock, maple, oak, pine, poplar, rose, spruce, tamarack, willow [list taken from Handfield, 1999]"
And how do they survive the winter here? Bug Guide says about the life cylce:
Here's a picture showing this moth on the four inch square tile so you can see its size."two generations per year in the south; one generation in the far north; overwinters as a third-instar or fourth-instar larva, likely exposed on bark and branches"
You can tell a moth from a butterfly by the antennae. Moths have feathered antennae (see top photo) and butterfly antennae are plain with a little knob on the end. Enchanted Learning lists some other differences.