You want to emphasize the important points. There's no doubt that the blue and yellow above contrast. Not only do they contrast in color, they contrast in feeling, too. Blue is a cool color and yellow is a warm color. I think that there's little doubt that the final logo is more interesting than the original logo, even though it's not really a finished logo.
Contrast is what makes it a visually stimulating design.
Hopefully by the time we're done exploring design basics and have applied those to our Tennis Group logo, we'll have a more exciting logo. Ask them to design seven logos based on the original design. Have your students create designs using illustration board and contact paper. Have them create designs that illustrate the concept of contrast using these basic shapes. Then have the entire class discuss and critique the resulting designs.