Mask Background Color maskbg

When 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, 0FFF.

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.