Image.Mask (gb.image)
Function Mask ( Color As Integer ) As Image
 
Multiply each color component of each pixel by the color component of the specified color.
For example, if the red component of 
Color is 
0, the red value of all pixels will be cleared;
if the red component of 
Color is 
255, then the red value of all pixels will be unchanged;
if the red component is between 
0 and 
255, the red value of all pixels will be reduced proportionally.
In other words:
Pixel component = Pixel component * Color component / 255
The same process occurs for the blue, green and alpha component.