Whichever you click will go right in, then it'll ask you for another for that specific chemical holder, as the beakers can need more than one kind.
I should have made it clearer, just didn't, all I can say is if it's one beaker that you want more than one in just make sure you hit the right color then hit yes to add the second color to the same beaker, and then hit no to go to the next beaker.