Glass bottle bricks: Eco friendly colour options

Hiya, folks. Greetings from South Africa.
I've collected a whole bunch of glass bottles: Clear, green, brown. I'm looking for ways of colouring the clear ones red, blue or yellow. I've read that they can be coloured with resin, but as far as I know, most resin is not eco-friendly, and the good stuff is most likely hard to find here.
I'm thinking of painting the tinfoil covering the outside of the glass bottle bricks with ink on the inside. Has anyone tried this?

Welcome to Permies, Clare!

When I was a child, we did a school project where we brought a maynaise or peanut butter jar to school, glued things to the inside that we cut out of magazines and then painted the balance of the inside with some kind of paint. I'm sure in the mid-70s, no thought was given to eco friendliness, but I'm sure you can adapt the process.

But your last line about the tinfoil leaves me unsure that I understand your project.

As for resin, you can collect it on a knife edge from fallen pine limbs, thin it with a solvent -- maybe ethanol(?) and tint it with minerals like powdered lapis or clay or whatever.

Maybe paint the inside of the bottles with milk paint.  The problem I see with this is that when the bottles are heated to make the bricks the paint might burn and turn black.