Principles of design are rules that apply to all types of design, and when it comes to interfaces, these rules can make websites and apps more usable and more accessible.
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Contrast defines how easy it is to notice certain design elements.
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Emphasis increases contrast, i.e. it makes elements more noticeable, whereas balance decreases it, i.e. it makes elements less noticeable. Contrast is defined by color, size, and depth, so bolder colors, for example, will add more emphasis and therefore more contrast, whereas not-so-bold colors will add more balance and therefore less contrast.
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Sometimes, we might actually want to decrease contrast to reduce noticeability.
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