Paint tubes are sometimes a waste. When a person finishes painting with a plastic tube they will throw away the tube once the paint has gone. The tube is such hard plastic that it will take more than 600 years to break down. You could make your own paints instead of throwing away junk that will eventually end up in the sea or land fill. See this link for a great recipe: http://www.kids-crafts-fun-and-games.com/homemade-paint.html
Another idea if you need to buy paint tubes is to recycle them to make chalk. See: http://playfullearning.net/make-your-own-sidewalk-chalk/. Use the tubes as molds for the chalk. You could also fill them with sand, replace the lid tightly and use them as wedges to hold doors open.