This is the 1634th most frequent Russian word.
белом
"Белом" is the prepositional case form of "белый," meaning "white" in reference to something.
The word 'белом' is used here as a prepositional form of the adjective 'белый' (white) to describe snow as a white medium.
Я вижу птицу на белом снегу.
I see a bird on the white snow.
The word 'белом' specifies the color attribute of the dress, indicating the shade of white.
Они подарили ей платье в белом цвете.
They gave her a dress in white color.
The word 'белом' is describing the white sheet in terms of its coloration.