Generally most varieties of snowdrops are white with some green blotches on the inner petals. But some have a hint of green and a few have a slight hint of yellow. My personal preference is to preserve the white of the paper for the white areas of the petals (really only seen where the flower has been lit up).
In watercolours you can mask out the white using masking fluid. Alternatively, you can opt to paint the white flowers with titanium and zinc white. Zinc white should help you create translucent flower heads that merge into the background and create recession and depth in the painting. Of course there will be nuances of shadow colour on the white petals. These need to be handled delicately with subtle shades of grey to avoid making the petals too heavy.
DEMONSTRATION - Carpet of Snowdrops