As of November 2015, here’s how to do it with one line of code in console:
mv /Applications/Dropbox.app/Contents/Resources/emblem-dropbox-uptodate.icns /Applications/Dropbox.app/Contents/Resources/emblem-dropbox-uptodate-back.icns
Then you just need to restart Finder. You can do this by opening Finder and choosing “Force quit” from the Apple menu.
The Cnet instructions didn’t work for me.