For what purpose? Candles, food??
These ones look cute
For decorative purposes, Pour some paint in and swirl it around. Turn it upside down and let it drain out, then turn upright and let it dry before using them. You could also put a little of more than one colour in and maybe swirl or use a stick or anything else to hand to spread the paint to give desired effect. Have fun......
I painted some jars to make spring decorations and this might help you get started - https://chascrazycreations.com/recycled-jars-to-bunnies-chicks/
Hi Green, You can also paint the outside only if you're wanting to use the jars to store food items. Just paint the outside with craft paint using a sponge or brush. Let dry and then seal the paint well with a clear coat sealer. The jars can be hand washed to clean them. You could also use ModPodge to add a design or monogram after painting and before sealing.