I have to agree they are under exposed. What type of camera did u use. All camera's meter the picture by taking all the colors in the photo and mixing them up to achieve middle grey, which would work fine for most pictures that have a lot of colours in them (grass,buildings,trees,etc) your photo fools the internal meter by taking all the colors in your photo (mostly white) and making it middle grey. You as the photographer need to understand this and compensate for this by adding 1 to 1.5 stops of light to the photo to make the snow white. one easy way to do this is reading up on exposure compensation and adding +1 or more to the over all photo. You would do the oppostie in the case of a mostly black photo minus 1 to 1.5 stops of light to make blacks black. If in manual mode over expose for the snow or under expose for the blacks if that is a predominate color in the photo. Other ways of doing this just thought this might be easier to understand. Get out there and try again dont be afraid to mess with the settings.