Take your jars and put your desired food colouring and water in.

Add a squirt of mod Podge. Then stir together.

Rotate jar to evenly coat the inside with the mixture.

Place jars in a warm oven to dry them off.

Becoming clear.

Two colored vases ☺

I've only tried this out once and had a few issues with some areas being darker than others etc. so it's trial and error really to find out what works best. And I'll give it another go soon!