This is only superficially
trivial.
Ice is
transparent.
Snowflakes on the other hand are not solid ice but very
complex crystals. Therefore there are so many
reflections and
refractions going on in a pile of snowflakes that incident light is
scattered back in the end. So far, so good.
Now here comes
the catch.
Sunlight is not white, but yellow.
Well, actually, it is
white but it gets yellow because of
Rayleigh scattering in the
atmosphere. See also "
Why is the sky blue?".
So why does the
snow not look yellowish?
By the way,
Webster 1913 defines
white as "the color of pure snow", so we might be running in a circle here ;) In my opinion that's a bad definition because it does not mention what kind of light is shining on the snow.
But anyway.
The point is that our
brain makes it so!
It apparently compares the light going in with the
light going out. If it's the same
color, the object
is white. Therefore white
paper looks white to us
even under colored light - but when you take a photo,
it is definitely colored as well.
So why is the snow not at least yellow on
photos?
That as it turns out, is entirely due to the
film used! It is designed so that "white" objects in
sunlight look white. This as also the reason
why one has to watch out when using a
digital camera,
because the
CCD in cheaper models often has issues
with the
color balance rendering the snow yellowish!
Update:
wrinkly points out that the
blue of the sky and the yellow of the sun might combine to white again. I cannot refute that. However, it is still true that snow can look yellowish on a bad photo, so apparently it does not quite add up!