Juliana Have you considered colored oils or liquids in the jars?
I've only seen these once and have no idea how they would work or how long they last.
But different oils of different weights will form colored layers. I had a girl friend who made and sold flavored herb flavored oils and vinegars and her multicolored oils sold like crazy.

I wonder if this would work as garden art?

I have used them as candle holders. You can wrap a thin gauge of wire around the rim then attach 3 or 4 other wires to that wire, bringing them straight up and joining them to creat a hanger. Wrap a short piece of wire around the top of the three pieces to bind them. Curl the ends of the top pieces into curly-cues. We have hung them from tree branches and put in small candles. It looks cute at night.

I like the idea of candle holders. group them together to form a chandelier. YOu can add beans or something decorative inside the glass. or you can use coloured chinese paper to wrap outside the glass.