iOS also provides semantically defined system colors that automatically adapt to both light and dark modes. A semantic color conveys its purpose rather than its appearance or color values. Primary for the overall view, Secondary for grouping content or elements within the overall view, Tertiary for grouping content or elements within secondary elements. 
- Don’t redefine the semantic meanings of dynamic system colors.
- Don't try to replicate dynamic system colors.

University of Michigan - Ann Arbor

iOS offers a range of system colors that automatically adapt to vibrancy and changes in accessibility settings like Increase Contrast and Reduce Transparency. The system colors look great individually and in combination, on both light and dark backgrounds, and in both light and dark modes. Don't hard-code system color values in your app. 

iOS: System Colors

Apple Human Interface Guidelines

iOS 13 also introduces a range of six opaque gray colors you can use in rare cases where translucency doesn't work well. For example, intersecting or overlapping elements — such as lines or bars in a grid — look better with opacity.

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