Sometimes it's hard to know where to start when painting snow. Using a lot of white just will not work.
Titanium White acrylic paint is an opaque colour; this means that you can over-paint with the white to add lighter areas. This takes away a lot of the worry about painting the 'wrong area' and not being able to fix it!
When starting to paint snowy areas I like to start with a medium tone.
In my example I started with a little Ultramarine Blue and Titanium White mix. Vary the tones a bit when you paint the snow. Think about any hollows and make them a bit darker (bluer). I like to use blues in snow scenes, I think the cold look it gives is appropriate for the subject. However you can also use other colours depending on the result you're after.
In other paintings I've used a brown (Burnt Sienna) instead of blue. It works well and gives a warmer look to the finished picture.
In my example you can see that I have some warmer tones of brown in the tracks in the foreground, this helps to add depth to the painting (warmer colours to the front). I also added some purple tones for a bit of variety.
Snow will reflect other colours so if you were painting a sunset or sunrise you could add some of the sky colours to the snow. This will add a unity to the final picture and it's fun to be able to use all the colours too. And, as before, if you paint on too strong a colour you can always pale it down by over-painting with white.
Save the pure white for highlights. You can paint the highlights several times to cover up any existing colours and to get a bright result.
I find it best to leave the completed painting and come back to it the next day and add the highlights, and often I darken a few areas for contrast too.
Doing this really gives the picture a lift and makes it sparkle.