Several years ago I had an issue with an unmarked bottle I purchased as an empty bottle and filled with liquid. The bottle was tiny (IIRC one ounce) and clearly marked with it's volume. The screener was upset that it wasn't an original travel size bottle of shampoo, mouthwash or whatever and said something about not being able to know what was in my unlabeled bottle. I avoided pointing out that she couldn't know anyway unless the bottle was factory sealed.

Since then I recycle hotel toiletry bottles and any sample/travel sized ones I get. As long as the liquid of a well know brand label isn't obviously of the wrong color for the label of its contents, how can the screeners know?