Gets the border color of the selected image. It is a value specifying the 32-bit RGBA value.
If it fails to get the border color, it returns
Default is transparent (0x00000000). The byte-ordering of the 32-bit RGBA value is RRGGBBAA. RR represents red, GG represents green, BB represents blue, AA represents opacity, ranging from 0 to 0xFF. 0 is transparent, 0xFF is non-transparent.
In browsers, opacity is from 0 to 1. We will transform it, so sometimes the value set and get will have a small difference.