The first image was painted using a single color and varying values to suggest the volume. Loose sketches like this become easier with practice and helpful in thinking of the object as a whole without fussing over details. Yes - those are a couple of my bags.
The middle photo shows demos of the assignment including a paper bag and simple boxes. I use a light violet wash and paint in my shadows first. After that I cover the box with a color, and build up a pattern with a third wash. This helps lock in the shadows and it will help the painting hang together. The background is often done last, because I make some up as I go and ask for color suggestions from the class.
My recipe for 'paper bag brown' = cadmium yellow with dioxazine violet. If you don't have that, then try gamboge or another warm yellow. Prussian or thalo blue with permanent rose also makes a lovely clear purple. In any case, the purple is a "cool" neutral shadow color. As you look at paintings you will see how often purple plays a part in the shadows.
The last illustration gives step by step guidelines for painting this box. Feel free to use it and experiment with making the box different colors and shapes. Notice how the violet becomes a neutral under the yellow paint.
I hope these samples help you in your painting and or understanding of 'how did they do that".