The Tiffen 4 x 6" 3 Cool Blue Soft-Edge Graduated Filter helps to specifically add a cool, purplish-blue coloration to skies and other bright areas of an image while allowing a portion of the image to remain neutrally colored. The filter's coloring is densest at the edge and tapers to clear by the middle, with a soft-edged transition between the density and clear areas. The 3 color density provides less cool blue coloration than the 4 density, but more than the 2 density filter.