When saving an image as PNG in GIMP you are facing the choice of checking option save gamma or not. When you check it — according to official GIMP documentation — the gamma value of your display environment is embedded into the PNG file. I was resizing the Gentoo logo, file gblend.png. Checking save gamma on export results in the left (correct) image, leaving it unchecked in the right: too far towards purple.
If you use Firefox or Thunderbird these two images look different. To my recent surprise they look identical in all of GWenview 4.6.2, gthumb 2.12.3 and Thunar 1.2.1. Interestingly, now that I have seen the difference I notice that the logo shown on gentoo-wiki.com does not have correct gamma. See for yourself. I have contacted its operator about it, let's see what happens. I don't really have time to inspect this issue much further. If you do that would be awesome. Please send patches to upstream for me. Thank you!
EDIT : Mike has updated the logo on gentoo-wiki.com by now.