If you want to replace all of the RGB values in the source image with a mask color and just use alpha values, you must provide both the color you want to draw and the background color you want to multiply the alpha with.
ColorMaskOptions
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If you want to replace all of the RGB values in the source image with a mask color and just use alpha values, you must provide both the color you want to draw and the background color you want to multiply the alpha with.
ColorMaskOptions