In the scenario you described, contrast in the image could be created by using complementary colors. When colors that are opposite each other on the color wheel are used together, they enhance each other and create a striking visual contrast.
Additionally, using darker and lighter values of blue can also help create contrast within a monochromatic scheme, emphasizing different aspects of the water ball and the droplet.
If you have specific artistic techniques or elements you want to discuss further, feel free to share!