You can actually fill them with a lot of different things.

But paint is great if you want to make an avant garde art piece commenting on the prevalence of surveillance in a community, or rejecting the transparency of a given surface.

If you do this I suggest 2 points.

Do not use the original cap thing, it will leak, you can plug it with tape then cap it, corks risk breaking it.

Use padding to store them. The extra weight and fragile glass makes them more likely to break in transit.