just seen this post, so here goes.
i have been wondering about this myself, and all i can suggest is to make the colours they have a bit stronger - perhaps have just the smallest bits of very dark green picked out as patterning to represent the evergreen's leaves piercing the snow.
if you manage to work something out that looks good, post up some pics - winter themes are very popular with wood elves, and a bit of variation on the theme would be good.
as for the lack of cavalry for a winter force, i would disagree with your example, you could just give the stag a red nose
what you should avoid from a fluff point of view however is taking too many forest spirits. a couple of small dryad units for support, but no treemen or tree kin, and definately no orion or wild riders.
a unit or two of eternal guard would be a good start however, but an expensive one!