Snow will always show up gray if you let the light meter choose the exposure.
Here is what you need to do, With your camera in the manual mode, set your shutter speed fast enough do you don't get camera movement when you take the shot (1/250th is good). Once you have used the correct aperture set, open up one stop to let more light in so that the snow is white.
If your camera has an EV adjustment option, you can set it to +1 and leave it there until you finish shooting the snow ... remember to change it back to 0 when shooting normal subjects.
OR ... the simplest may just be to manually meter your hand (out of focus is fine) and then use that meter reading.
Here is what you need to do, With your camera in the manual mode, set your shutter speed fast enough do you don't get camera movement when you take the shot (1/250th is good). Once you have used the correct aperture set, open up one stop to let more light in so that the snow is white.
If your camera has an EV adjustment option, you can set it to +1 and leave it there until you finish shooting the snow ... remember to change it back to 0 when shooting normal subjects.
OR ... the simplest may just be to manually meter your hand (out of focus is fine) and then use that meter reading.