When I save an image that uses transparency and pale colors, to the ICO, IrfanView (4.52 and 4.53) mixes these pale colors together with the main window color (used to show transparent parts). So the only way to get the same image as the PNG source, is to set pure white background color.

Source PNG: Name:  source.png
Views: 80
Size:  703 Bytes

Resulting ICOs, in accordance with the window background colors: ico.7z

If I edit and save transparent PNG files, colors are not mixed, and the result looks as a source, regardless of window background color.

Is this a bug, or a side effect of ICO images processing?