If they are the ones I'm thinking of, take the cap off and pour paint. Thinned down inside and swirl around.
I would spray paint them.
This idea using nailpolish looks easy.
Fast forward to 38:00 minutes where the demo begins.
Hi Elizabeth, You can just pour paint into the ornament, roll around to cover the inside and then place upside down until dry.
Here's a post I did about it :)
Pour some paint in and swirl it around and let it drain dry. ........
I have in the past painted with testors enamels or acrylic paints.