Why is my battery yellow in charging mode?

It's because you have turned on "Low Battery Mode" before you started charging, it will turn off automatically after charge reached 80%. When your battery is below 20%, there will be a notification if you want to turn on "Low Battery Mode". You might have turned it at that time.

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

Patricia

Because you've enabled the Low-Power Mode.

Disable it under Settings > Battery.

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