I use the same dropper for putting thinner from another container... Its a self cleaning process.. This is how I do it.
1: Take out paint from the bottle.
2: Put thinner in a seperatre container to avoid contamination.
3:After paint has been transferred use the dropper to take out the thinner.
This way the dropper is automatically clean. You may want to clean it further using water.
Cheers,
Nandakumar