According to Deb Menz in her wonderfully understandable book on color theory, Colorworks,
"A white background tends to make all colors a little darker, duller, and cooler."
Interesting, since I think our instinct tells us that high contrast automatically intensifies color. But instinct can be wrong.
Perhaps, in the case of snow, the contrast simply makes us notice what we normally do not see at all. I never saw these fruits before on my daily walk. They are exactly the sort of rich color I love, a deep purple red that is almost black....the stems are a brighter version of the same color. Go ahead.....click on the picture for a closer look.