FireWorks is an editor which can let you to optimize your PNG files, by reducing the number of colors in the RGB area, keeping or not the original PNG ColorType (choose to keep in PNG32 or choose PNG 8 with alpha values).
FireWorks can reduce the number of colors in a PNG32 file. It can give you a better compromise between quality and space. To do that, just import your file.
Use Filters, deploy Adjust Colors, and use Levels.
Replace the 255 value of Output values by 64 (or lower), then click on OK button. Your image should be more dark. Then, you have to do the same : Filters menu, choose Adjust Colors, and use Levels.
This time, modify the 255 value of Input values by 64 (or lower, but equivalent as your precedent input), then click on OK button. Choose File, and Export.
Fireworks reduced the number of colors in the RGB area of your PNG, but you can optimize it further with a more advanced compression tool, as pngXoptimizer or ScriptPNG. In this sample, i used pngXoptimizer. Download it, decompress the ZIP archive, and just Drag-and-Drop your PNG files ON the pngXoptimizer executable.
| Original File | FireWorks | pngXoptimizer |
 |
 |
 |
| 13232 Colors | 5294 Colors | 4556 Colors * |
| 45,7 KB (46 876 Bytes) | 36,6 KB (37 495 Bytes) | 29,3 KB (30 084 Bytes) |
* Please not this procedure is lossless for Web (the pngXoptimizer process), even if colors are reduced, because pngXoptimizer destroy all non-displayed
values in the RGB area. For more informations, see PNG Transparency - RGB Filtering.
Import your PNG32 file, then choose PNG 8 in the Optimize windows.
If it's not automatically selected, choose Adaptive, 256 colors, and Alpha Transparency. If it's necessary, you can adjust the Dithering (which is a diffusion method used to give a better image restitution for some files). Then, choose File, and Export.
As the precedent sample, your PNG could be optimized further by using a better compression tool.
| Original File | FireWorks | pngXoptimizer |
 |
 |
 |
| 3465 Colors | 148 Colors | 148 Colors |
| PNG32 | PNG 8 | PNG 8 |
| 38,9 KB (39 891 Bytes) | 13,6 KB (14 023 Bytes) | 13,0 KB (13 379 Bytes) |