Creating differences between elements through color or variation in size and font refers to which design principle?

The design principle that focuses on creating differences between elements through the use of color, variation in size, and font is contrast. This principle is essential in visual design as it helps to highlight and differentiate elements within a composition. By using contrasting colors, sizes, or fonts, a designer can guide a viewer's attention, make important elements stand out, and enhance the overall visual hierarchy of the design. For instance, using a bold font for headings in conjunction with a muted color for body text draws the viewer’s eye to the significant parts of the content, promoting better readability and engagement.

While other design principles, such as balance, unity/harmony, and emphasis, contribute to effective designs, they do not specifically focus on the notion of differences between elements. Balance involves creating a sense of equilibrium in a layout, unity/harmony refers to the coherence and consistency throughout the design, and emphasis highlights particular elements to attract attention. Contrast is distinct in its ability to create visual differentiation, making it the most suitable answer in this context.