Snow scenes require some exposure compensation, particularly if you're not using full manual mode. Digital cameras in particular tend to darken the scene, trying to compensate for the brightness. This usually results in snow that looks greyish.

How much compesation depends on the scene and camera, but a +1ev is a good starting point. Take some test shots, and check the histogram. It's important to make sure you're not clipping the highlights.

Shoot in Raw, if possible, as this will allow you to change the exposure and white balance later, if neccessary. You'll probably have to readjust the settings for the indoor shots, so don't forget to check the settings again indoors.

Abover all, have fun!

- Joe U.