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.

You can indicate that an error occurred using haptic feedback. It can be useful for long-running tasks that fail, like page loads or downloads. But you can also use it as errors occur like Twitter does when reaching the character limit.

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.

Convey important information in multiple modes (sounds, haptics, colour, iconography, messaging...) so no-one misses it. Take Spotify's shuffle button. It is green when on, white when off, but it has also a dot indicator. https://wearecolorblind.com/examples/spotify-shuffle-and-repeat-buttons/