Spoiler:
May be someone from you knows about pngquant.exe. May be someone from you even it uses.
Pngquant is a command-line utility to quantize and dither 32-bit RGBA PNGs down to 8-bit (or smaller RGBA-palette PNGs, usually with a significant reduction in file size. (Other PNG image types, even colormapped images, may also be processed, but they will first be expanded to RGBA, which isn't the most efficient way to do so.) This is the same technique used for many of the images on the Miscellaneous Transparent PNGs page, and the results are often indistinguishable from the original, truecolor PNG images.
Color Depth Changer v.1.0.exe uses for the work pngquant.exe.
Color Depth Changer v.1.0.exe is an instrument which will help you quickly and very qualitatively to transform quantity of colors in the PNG image to necessary value (in normal amount PES6 of 256 and 16 colors). For this purpose it is just necessary to place the PNG image which we want to transform to one folder with Color Depth Changer v.1.0.exe, start Color Depth Changer v.1.0.exe, choose desirable quantity of colors in the image and to press Enter. If all of you have made correctly in same folder to appear the changed image with the same name as an initial file with addition "-fs8". All is ready. Quickly, qualitatively and simply! It is possible to process more than one file at the same time.
Enjoy yourselves! I wish you pleasant game!