A green battery icon on an iPhone indicates that the battery is charging. When you plug in the charger to the iPhone the icon changes from white to green.
In some cases, it may change from yellow to green. If the iPhone battery icon is yellow, it means that your device is in low power mode. This is a power-saving mode that activates when your iPhone falls below 20% battery life.
The icon may be black instead of green if you move to screen like the settings menu where there is a light background. It can turn white if you’re on a screen where there is a dark-colored background (see here).
As your device charges, you’ll see a lightning bolt beside the battery icon in the status bar, or a large green battery icon on your Lock screen.
- Status icons and symbols on your iPhone | Apple Support