At a christmas lunch there was a table set to paint the clear christmas ornaments, I took one home to paint as a present. I used black and silver paint inside to swirl around, and when that dried I painted snowflakes on the outside and used a glitter paint I had over the snowflakes.
Take your ornament and pour in some paint, let the extra drip out and after it's mostly dry then add in the next color. Again let the extra drip out and keep repeating until you are happy with it.
Once the inside is dry then paint some snowflakes on the outside, or any other christmas design you like.