One of iOS 18’s many new customization features is the ability to uniformly tint the color of your app icons. But the latest beta revealed another nice change: both first-party and third-party app icons will automatically change when in dark mode, without developers needing to do a thing. Thanks to some digging from Gui Rambo, we now know how that works.

Automatic dark icons for third-party apps

Gui Rambo writes on Mastodon:

In case you’re curious, I’ve checked the implementation for icon segmentation in iOS 18 to see how they’re generating the dark icons. There’s no ML/AI involved, it’s all just clever math. IconServices extracts the foreground and determines whether it should be tinted with the background color or left as-is, and there are configurable thresholds for when the results are not acceptable, which is when it falls back to just darkening the icon. Looks like it uses ARM Neon instructions for speed

Users running the latest iOS 18 beta may have noticed that not only do third-party app icons automatically change in dark mode, but they do so in a couple of different ways.

Some icons, such as that of YouTube, turn the icon’s normal background black, and even make changes to the icon’s primary color. This is especially interesting in cases where the icon’s background isn’t white. For example, Facebook’s icon turns the background a darker blue and changes the ‘f’ from white to a light blue.

The other way icons change is by simply applying a dark tinting, but otherwise keeping their colors in-tact. A good example is Instagram, which apparently doesn’t meet Apple’s threshold for changing colors altogether.

The general rule seems to be: if an icon features a fairly simple two-tone color approach, those colors will likely be inverted or otherwise changed when in dark mode. But if it has a more complicated mix of colors, like Instagram’s icon, a simple dark tint will apply instead.

Now thanks to Rambo’s digging, we know that there’s no complicated AI magic happening here—instead, it’s a simple case of checking the icon’s background and foreground colors against simple mathematical thresholds, and dynamically adjusting accordingly.