Just set the Alpha level in the RGBA setting for the color of the control. The code will look something like this:
Control.Backcolor = Color.FromArgb(255, 255, 255, 255)
The first value passed to the FromArgb method is Alpha. A high value will mean high opacity, while a low value will mean high transparency. You may also need to set the Forecolor property of the control if you also want it to be transparent.
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