Unless you take control, when your camera looks at a bright snowy frame, it thinks – whoa! That’s too bright! I need to darken it down!
And so it does, making your snow turn out gray.
Fix this problem on phone shots by touching the screen to set your focus, and looking for the little sun or light bulb on a line beside or below the focus square. Use your finger to slide up or down on the line to increase exposure until the snow is white, then shoot away.
It’s more complicated on a regular camera – you need to understand how to control the exposure using ISO, shutter speed and/or aperture – let me know if you want help on that.