Software for lowering the colors in PNG-24 files

There are two tools:

• pngquant which creates 256-color PNGs with alpha channel. That greatly reduces file sizes (60-70%). In latest versions it may have better quality and smaller file size than Photoshop's Save For Web (even for images without alpha).

• mediancut posterizer reduces number of unique colors in 24/32-bit PNG files. This can reduce file sizes by 20-30%, and again is smarter than posterizing colors in Photoshop as it also posterizes alpha channel and is smarter about selecting posterization levels.

Both of those programs have a GUI: ImageAlpha.

You could use ImageMagick's convert application with the -color flag.

http://www.imagemagick.org/script/command-line-options.php?ImageMagick=fv5ffdto8nt4revt54g3ba7ne7#colors

i.e: convert something.png -color 256