@NeoHomeBrew:
thanks, your latest tables are implemented, but the difference between linear- and save-colors is only noticable with gadients. It's implemented as option, switchable on the fly.

I do not handle the 32 dark-bit colors , only the non-dark-bit colors - there is a simpler solution: COLORVALUE * 2 + 1 - DARKBIT = value from table.

NEW importwindow shows both images, original (left) and processed (right). it is resizeable and zoom with mousewheel is supported, swap colors by using drag-and-drop.
there are 4 simple modes for color reducing:
-> 16colors
-> 256colors -> 16colors
-> 16colors (dithered)
-> 256colors -> 16colors (dithered)

sorry, only one palette (16colors) per fix/spr but I understand what you mean - what you request is more like a very special paintprogram.