Bill often used two brushes: one for light paint and one for dark paint. That saved him time so he didn’t have to clean the brush when he switched colors.
Notice in this clip how Bill corrects his mix. We’ve seen him do this before. He doesn’t get frustrated or angry with himself, he just fixes the color to get it the way he wants.
Also notice how he loads his fan brush. He’s not jabbing it into the paint the way you see a lot of amateur artists do on YouTube. He gently “flips” the brush while still getting plenty of paint on it.
A couple of great tips on this clip!