How to reduce color depth to a specified palette?

[scurest]

Is there a way, in any program, to specify an allowed palette of colors and automatically reduce the color depth of an image to that palette?

I know you can do that in mtPaint.

You'd crop out one monster, do "Image > Convert to RGB" if necessary, then edit the palette until it has the colors you want. Then you can do "Image > Convert to Indexed" and use the "Use Current Palette" option with Dithering set to "None". Then assemble monsters into one sheet afterwards.