Reduce Number of Colors in a PNG image

You can reduce number of colors in a PNG image to reduce PNG size. This is a lossy methods ; that's mean the output file is NOT identical as the original. This technique is very effective on RGB+Alpha Files.

Using ScriptCQ

ScriptCQ is a Windows Batch Script which can reduce number of colors in a PNG image automatically. Just Drag-And-Drop your PNG Files on the ScriptCQ.cmd batch file, and follow instructions :

• ScriptCQ proposes 7 Posterization modes : 90, 80, 70, 60, 50, 40 or 30. It represents the approximative purcentage of colors which will be present in the optimized image.
• It optimizes the output file with a good filtering technique, and a powerful 7-zip deflation algorithm. However, the output file obtained with ScriptCQ can certainly be optimized further with ScriptPNG.

Reduce Colors in PNG32 (RGB+Alpha)

This PNG sample was converted with PNG32 Quality 30%, option 7. It was proceeded in 2 seconds with a Celeron processor, including the posterization, an optimization process and a fast 7-zip recompression.

PNG32 Quality 30%
13 232 Colors	1948 Colors
46 890 Bytes	23 596 Bytes

Convert to PNG-8 with tRNS

This PNG sample was converted with Convert to PNG8, option 8. It was proceeded in 4 seconds with a Celeron processor, including the posterization, an optimization process and a fast 7-zip recompression.

Convert to PNG8
3465 Colors	192 Colors
39 891 Bytes	13 701 Bytes

Using Color Quantizer

Color Quantizer is a tool which can let you to reduce number of colors in a PNG file, or convert it to PNG8. You can download it here : Color Quantizer.

Just import your PNG file in Color Quantizer, and select the approximative number of colors you want in the RGB area. Your PNG will be posterized on the fly, you can then control the output quality.