All I can think of is to have the gas in a (clear) balloon inside the bottle.
Note that fluorine will react with glass (it will actually react with just about anything)
Helium is very hard to keep contained (it will happily seep through what would normally be considered "airtight" gaps).
Many elements (particularly gasses, and many metals) are visually very similar.
I assume that you will be stopping before you get to the radioactive elements...