Inside paint bottle with mod podge and food color
Susan: maybe this will help
This is how I painted inside glass ornaments but the same process would work for a bottle - https://www.charlestoncrafted.com/diy-sea-glass-ornaments/
Hometalk has a lot of posts about various ways to do this:
You can create a transparent color look on your clear glass bottles by mixing food coloring and school glue which dries clear. Mix the glue and color together until you like the color, pour into the bottle and turn the bottle various ways to get good coverage and then pour the excess out of the bottle and let the bottle dry. Fun!