PNG images colors are effected when shown in a web-browser, if you copy the image from a website and save it as PNG, put it up on the web and repeat multiple times the effect is cumulative.
Whats happening with the colors here has to do with the emulators you're using, most handhelds have slightly different colors than computers so each emulator has it's own filter for converting them, and they might be different.
EDIT: Oh, wait, you said you're both using No$gba? What are each of your settings for "GBA Mode" under Emulation Setup? That sets the color filter. If I had to guess, I'd said Desgardes is using VGA mode, and Linky's using one of the other ones (They all seem to effect DS images the same way)