Int 10 Fn 101B  - Video - Perform Gray-scale Summing (vga/mcga)            [V]

   AX = 101Bh
   BX = starting palette register
   CX = number of registers to convert
Desc:  convert the RGB values of one or more palette registers such that the
     resulting values are grays with the same intensities as the original
     colors

See Also: AH=12h/BL=33h