The color not to be drawn in the button's image.
StringValue can be any string meeting the following format requirements:
Syntax: [RGB Color | "Auto" | "None"]
This property allows the developer to specify that a certain color in the button's image not be drawn. This allows bitmap images to appear as natural irregular shapes rather than as rectangles.
The easiest way to make a color transparent is to set this property to Auto. In so doing, the control will always use the image's top left pixe, the pixel located at coordinate (0, 0), to determine the transparent color. If you want a different color to be transparent, then pass an exact RGB color value. You can do this using OI's RGB function.
This property is ignored if the button's image was loaded from an .ico file since icon files already contain the transparent pixels.