When I looked around for a way to do a snow basing, I eventually settled on this effect:
That's my Terrorgheist, and the Snow is made by taking Gale Force 9 snow powder stuff, and mixing with PVA glue and a little bit of water.
The mix is kind of hard to get right the first time, so I had to experiment a little, but I think the effect came out great.
The pros with this is that you're able to make the snow thick, like actual piles of snow, and the snow will be hard. Yes, it is actually pretty hard and rigid, even though it doesn't look that way.
As for what to have under, I have some grass under the snow, but a light blue / light blueish grey would probably do the trick.
edit: Damn, ninja'd by n0t_u
Well, consider the advice seconded.