Board Thread:Questions and Answers/@comment-174.95.112.183-20130405170116/@comment-83.109.225.68-20130405190212

It is the right click, unless you have been messing around with different settings. Just right-click on a RedPower2 tube (Not buildcraft, SOO many people mess that up) with a paint brush in your hand. And remember to dip it in paint by crafting the paintbrush next to a can of paint with your desired colour. The paint can will take a small amount of "damage" and the paintbrush will become usable.