I forgot that Dropbox changes the folder icon, based on the status. To change those, you'll have to modify the Dropbox.app.
First, make a copy of the Dropbox.app, in case you damage something.
Then go to /Applications/Dropbox.app, right-click and "Show Package Contents"
Then go to /Contents/Resources, where you'll see the various icons.
Replace those with your choices and save the .app.