If you choose Tinted app icons rather than the Dark style, iOS applies a different algorithm to change the colors of icons. Tinted icons feature an accent color on a dark black background.

How-To iOS 18: How to change the color of the app icons on your home screen Light mode, dark mode, even tint icons to whatever shade you want.

This detailed guide with screenshots covers how to change iPhone app colors and theme in iOS 18 natively with a few taps.