This is from an old guide, but it explains masks:
WindowBlinds 2.0 allows users to change the color of skins. A skin author, however, may not want the entire skin's color changed. Instead, they may only want certain aspects of a skin changed. For instance, you might want your blue title bar to be able to be colored but not have your tray border. So how do you do this?The answer is you create a mask image. A mask image is typically a grayscale image in which white areas are areas you want the colors changed and black areas being ones you don't want changed (with intermediate colors being in between). If you are an expert on colors, you can get fancier and supply a 24bit mask image and come up with some really strange blendings (for instance, if you want only the the reds blended into your skins of a chosen color, you would supply an image with a color mask of 255,0,0 ). We only recommend that for die hards. A gray scale image is the most intuitive way of creating a mask in this instance.To create a mask, you simply take one of your existing bitmaps, turn it to a grayscale and make the areas you want changeable white and those you don't want changed black.

A rarely used and now mostly unnecessary feature because of the new colorize abilities in WB6.

Some skins may have decorations that will look ugly if colorized. It sounds pretty useful for more complex skins.