Editing WoW:API Frame SetBackdropColor (section)

{{widgetmethod}}

Allows you to set the color value, and alpha value used to tint the backdrop of a frame, as set by [[API Frame SetBackdrop|Frame:SetBackdrop]]([backdropTable]).

 frame:SetBackdropColor(red,green,blue,alpha);

There are special considerations to be aware of when using this method. Specifically:
*The color value used here is meant to tint the backdrop file used in the backdrop of the frame. Tinting does not and cannot change the base color values of the file used, and as such tinting a black backdrop will have no effect whatsoever. Tinting black will always result in black anyway.
*Alpha values do not change the base alpha value of the file used as the backdrop. This setting does not and cannot change the base alpha channel of the image, so any alpha setting you use will be a modification of any alpha channels already present on the base image. Setting the alpha to 100% on an image with an alpha channel at 50%, will merely show the image with 50% opacity.