Become a Picnik Master - Picnik Help: "Control color even in Black and White with Color Filters.
Color filters are traditionally used on the front of a lens to block some colors while allowing others to pass through in order to control what colors in your photo produce the darkest and lightest tones. Choosing the right Color Filter for the Black and White effect can accomplish this too, letting you digitally achieve the same level of control over your blacks and whites and shades of gray.
To keep a large blue sky from turning your photo drab, choose a blue Color Filter to turn the sky brilliant white in your photo. For portrait photography, Red filters work best to bring out faces. Experiment! Click and drag your cursor around the color palette to get a live preview of how the different filters bring out various parts of your image.
Color filters in action: The original color is left, turned black and white with a blue filter is center, and with a red filter, right."