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I am not an expert on using files in Indexed Color and the Color Tables though I have dabbled with them.  The answer is yes they can be customized and here is the link to the Adobe Help file:

Customizing Color Index Tables

Also, here is a link to a way that describes how to extract color tables which can be saved and used as the tables for Indexed files (see answer 1 in the link):

Extracting Colors out of Color Table

Now that's a lot of push ups yet it can be pretty accurate.

Here is an alternate approach using the techniques mentioned and combined with the attached image.

First Layer was the extracted Layer Mask with the Threshold Layer Applied - I also added a Layer style of Color Overlay with the Color set to White and the blend on the Layer Style set to Color.  This is a quick and dirty way to extract the luminosity out of the color image.

Second Layer is a Posterize Adjustment Layer set to a value of 9 (adjust to get the right level of gray levels desired

Third Layer was the original Layer with no Layer Style and Blend set to Color to add the original color back in.

This may come close to what you want yet you have a variety of options now from which to choose.

Hope this helps

John Wheeler

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