Adobe Animate.
compiledClipInstance.backgroundColor
Property; a string that specifies the matte color when Opaque is selected. This is a string in hexadecimal #rrggbb format or an integer containg the value.
The following example illustrates getting the backgroundColor property:
var bitmapInstance = fl.getDocumentDOM().getTimeline().layers[0].frames[0].elements[0];
bitmapInstance.backgroundColor = "#000000";