glIndexMask controls the writing of individual bits in the color index buffers.
The least significant n bits of mask,
where n is the number of bits in a color index buffer,
specify a mask.
Where a 1 (one) appears in the mask,
it's possible to write to the corresponding bit in the color index
buffer (or buffers).
Where a 0 (zero) appears,
the corresponding bit is write-protected.
This mask is used only in color index mode,
and it affects only the buffers currently selected for writing
(see glDrawBuffer ).
Initially, all bits are enabled for writing.
GL.glIndex()