The term pixel denotes picture element. Bpp or bits per pixel denotes the number of bits per pixel.The number of different colors in an image depends on this number. The number of colors equals 2^bpp.
So, with 6 bpp are available 64 colors (2^6), and with 12 bpp are available 256 (2^12) colors. It follows, that with 12 bpp are available 4 times more colors than with 6 bpp.