It's very easy to underexpose subjects in the foreground when the ground (both in front of and behind the camera) is snow-covered. It's better to blow out the white, snowy background in order to obtain "proper" exposure of your subject in the foreground. With this approach, you'll lose some details in the snow behind your subject. You may also lose a considerable amount of shadow detail in the background, but it's a better choice to make when shooting portraits.
In contrast, the brightness of a snowy background can cause details in your foreground to be washed out. This will create a silhouette instead of a portrait. If you're looking for detail in a portrait, fill the frame with your subject. Try eliminating as much of the snowy background as possible by adjusting your composition. You may also move your subject to a backdrop of trees to keep your portrait properly exposed.
Shooting a portrait while it's snowing can have beautiful results. Snowflakes in the air below cloud cover will naturally diffuse sunlight. While this may not be good for landscapes, this softbox effect is great for portraits. As the snow keeps falling, be sure to try out a telephoto lens. The long focal length will enhance the visual effect of the falling snow.
WHITE BALANCEWhite balance settings tell the camera what color temperature light source you're shooting in so it can create the appropriate color cast for each scene. Differences in time of day, geographic region and weather conditions can make a huge difference in the color temperature of your light.
Most cameras have several automatic presets for common light sources: Auto, Daylight, Shade, Cloudy, Tungsten, Fluorescent, Flash, etc. Many cameras allow photographers to take a custom white balance reading—you can do this by manually setting the camera's white balance using a sheet of white paper before you start shooting. You may also dial in a specific color temperature (measured in degrees Kelvin) to better match your scene, which may not be covered with your camera's presets.
The preset white balance options are pretty accurate, but you have to remember to use the one that most closely matches your light source and weather conditions. On sunny days with clear blue skies, it's common to see snow picking up a slight blue tint; experiment with the Cloudy or Shade WB setting to warm up the overall color and neutralize that blue color cast. If your camera allows you to adjust the color temperature in increments, you can either cool down or warm up your photographs. If you aren't enthusiastic about carrying paper around to set the color balance in the snow, you can try setting the white balance to its Tungsten setting instead.