Clean the Brush Before Each Mixture?

Doan Le says: “What do you do when you need to change colors? Do you simply change brushes, or wash off the previous paint with water?”

If the new color I need to mix is not a big color change, I can keep using the same brush, even if there’s already some paint in it. I factor the paint that's in the brush into the new mixture. If the color I want to mix is significantly different, I may need to remove most of the paint with a paint rag. If I must mix a fresh, pure mixture that’s different from what’s in the brush, I have to thoroughly wash out the brush in a plastic tub of water and then dry it off with the rag. Each of these water cleanings dirties up the water supply, so if I’ve only brought a small amount of water, I try to limit the number of deep cleanings.