In my experience as a Colorado resident, the answer to this question is, "Only a small percentage of the time." When snow faces directly at the sun it appears pure white. Most of the time it will be a warm shade of gray. On days with bright sun casting hard-edged shadows, the snow will be warmer in the light and colder in the shadows (as in the photo above).
The opposite occurs on cloudy, over-cast days with diffuse shadows. In the photo below, the color temperature of the lights is colder while the shadows will be darker and warmer. You can also see that the edges are softer on an overcast day.
I like to look at and paint snow because it lends itself to watercolor techniques. In particular, I like to start wet-in-wet with stucturing the whites and then proceed by adding the hard edges.
Recently finished the fall newsletter where I talk about painting skies. See my website for more painting info.
