If you want to use complimentary colours for any image or logo then I find http://www.cssdrive.com/imagepalette/index.php really useful, you just upload an image or give it the URL of an image and it will give you the colours used and light, medium and dark variations of the same colours.
For the Atlassian logo at http://www.atlassian.com/dms/wac/images/press/logoAtlassianPNG.png it shows you that the image is made up of:
#003366 - dark blue
#9fc71c - green
#57afe5 - light blue
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