Why does my battery show yellow instead of green?

You are in low power mode. See Use Low Power Mode to save battery life on your iPhone or iPad - Apple Support.

It's triggered (the "old school" definition, not as in angered 😂) by your battery getting lower as a way to maintain power to your device.

Because you are likely in low power mode. Try charging your phone.

Lower Power Mode is enabled.

You can deactivate the function here:

Settings → Battery → disable Low Power Mode.

You can read more about Low Power Mode here:

Use Low Power Mode to save battery life on your iPhone or iPad - Apple Support