A reminder that the more modes we use to convey important information, the more sure we'll be that it will be perceived by all our users. Consider a combination. of color, icons, messages, sound, haptics, animations, etc.
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Haptics helps you signal important information in multiple modes. For example, you can use a UINotificationFeedbackGenerator to reinforce some "warning" feedback. Twitter uses this when you are running out of characters when composing a tweet.
You should convey important information in multiple modes, not just color. If you are still required to do so, at the very least you should complement that info with other modes, like symbols, if the user requested differentiation without color.
Manual testing is crucial. And therefore, reducing friction to let you start your testing process can be a huge help. Selecting some accessibility shortcuts will do that, putting most of iOS' accessibility features at a triple-click of a button.
