I got a puzzle for christmas and put it together the week of new years.  I was pleasantly surprised at how much I was able to learn about color harmony and the principles of unity and variety.  It was a puzzle of James Christensen's Parables painting he may not be a true master but seems to know what is doing with the art business.  As far as the color harmony and unity/variety goes between the 1500 pieces there was a lot of color harmony and the only way to tell the difference in the pieces was the slight variety in texture.  Puzzles are cooler than I remember.