background
Solid background color, written as #RRGGBB or #AARRGGBB. Alpha comes first, the Android convention.
Set this or background, never both. A semi transparent value is flattened over white on the Apple side, because the App Store refuses icons with alpha.
Throws
Gradle
at task time when the value is not #RRGGBB or #AARRGGBB, or when background is also set.