Mask Background Color
mask is set,
maskbg specifies the background color that will appear in the masked areas.
maskbg works with all values of
mask. Valid color values are 3- (RGB), 4- (ARGB) 6- (RRGGBB) or 8-digit (AARRGGBB) hexadecimal values. The "A" in a 4- or 8-digit hex value represents the color's alpha transparency. The default is transparent white,
maskbg can also be used to create multi-layer effects along with
bg, which sets the background color for a transparent image like this Illustrator file:
See the API specification for more information about this parameter's value ranges and format.