This visual filter is used for creating transparent masks. The transparent regions are defined by pixels set to the color value specified by the Color property.
The use of this filter is now deprecated in favor of the BasicImage() filter that was implemented with the IE 5.5 browser.
Filters are defined in style sheets as if they were a function call with its arguments expressed as name=value pairs. This is not the typical way to define arguments so you should be aware of this anomaly when working with filters.
According to the latest MSDN documentation, this filter appears to have been renamed as MaskFilter().