My iPhone battery icon is yellow color when charging

A yellow battery icon means your device is in Low Power mode. This mode can persist through charging - if you need to disable it you can do so in Settings > Battery > Low Power Mode.

Cheers!

Sounds like Low Power Mode is enabled. Home Screen > Settings > Battery. If yes and the phone is charged 40-50% or more, turn it off. Green now?

If not, wait until the battery is charged 80% + and check again. The battery should be green automatically at 80%+ charge.