This isn't 12-bit color depth. It's a 12 color palette, so its color depth is x if 2^x=12 or

       ln(12)
       ------
        ln(2)

Color depth (or bit depth) is the number of bits used to indicate the color of a single pixel. So 24-bit color depth produces 16,777,216 colors, 12-bit color depth produces 4096 colors, 8-bit color depth produces 256 colors, and 4-bit color depth produces 16 colors. A 12 color palette is 3.58496250…-bit color depth.