2D Flour Sack - Line art and Colour

After doing my sketch layer I did my final line art on a separate layer, using the sketch layer to trace over. I first used black for my line art so I could easily see what I was doing. I still think my line art needs some work, as the volume and shape of the flour sack changes constantly.

I then added colour using the fill stroke in another layer, and a shadow to make the movement clearer. I chose brown for the sack as it is realistic, however I find it very boring. I also changed the line art colour to a darker brown so it would blend better with the colour of the flour sack.

In all honesty, I am not happy with my outcome of this task. I didn't pay enough attention to the volume and shape of the flour sack. I would like to redo this animation when I get feedback on it.