Returns the CLUT index to produce the specified gray, on a scale of
0 (black) to 1 (white) at the current screen depth, assuming a
standard color lookup table for that depth. E.g.

See BlackIndex, WhiteIndex.

No compensation is made for the screen’s gamma function. This is
just a handy way of picking a few grays for simple text and graphics.
This isn’t appropriate for images that need correction for the screen

When the screen is in 1 to 8 bit mode, the Macintosh OS always makes the
first clut element white and the last black. In 16 or 32 bit mode the
clut goes from black to white. These CLUT conventions can be overridden
by Screen ‘SetClut’, which makes a direct call to the video driver,
bypassing the Mac OS, allowing you to impose any CLUT whatsoever.

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