Battery icon shows yellow color instead of green and iphone only charges to 59%

Low Power Mode is On.

Charge the iPhone & wait until the battery is fully charged (100%).

The Low Power Mode will turned off automatically.

See this Apple article: Use Low Power Mode to save battery life on your iPhone

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

When the battery icon is yellow, it means that low power mode is on. Low power mode saves power when your device has low power. Just leave your device charging until it reaches 100%