Why does the battery icon shows as yellow?

The battery icon is filled with the color Yellow when the iPhone is set to Low Power Mode.

Open Settings ➡️ Battery ➡️ Low Power Mode: Set this to OFF and check whether the battery icon is in normal color.

Axel F.

Above answers your question. As for iPhone charging issues, what is the battery health percentage?

Settings > Battery > Battery Health & Charging.

A value of 80% or lower usually means the iPhone is headed for a battery swap.

iPhone Battery Repair & Replacement - Apple Support

Other charging issues can include bad cables, debris in the Lightning socket, and a failing or a bad charger.

Wirtza wrote:

iPad is 100% charged, low power mode is off in settings, icon is still yellow.

Force restart: Force restart iPad - Apple Support

Hold the third button press until the Apple logo appears, continuing to hold the third press past any sliders that might appear.

If that does not resolve this, or if the battery health is 80% or below, contact Apple Support.

Low Power Mode is enabled on the phone. Turn it Off

Settings > Battery > Low Power Mode > Off

iPad is 100% charged, low power mode is off in settings, icon is still yellow.