The principle of design variety states that multiple contrasting elements can capture user attention. This is based on the human desire to indulge and explore and the fear of missing out.
Unity is when elements work well together to achieve a common goal. Using a variety of icons across several buttons could capture more attention because each button would look different.
Balance refers to any number of elements that are equal. Similarly to unity, variety and balance aren't mutually exclusive, as a variety can communicate visual hierarchy.
Contrast is how different elements are from each other. Various elements don't have to be contrasted in every way to capture user attention - they can be balanced.
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