Why is the charging screen yellow?

Hi there,

Are you referring to Low Power Mode ? It won't charge your battery faster, read the attached document if so

https://www.apple.com/batteries/maximizing-performance/

A

Is the whole screen yellow, or the battery image yellow? A yellow battery image in the middle of the screen means that there is insufficient energy in the battery to operate the phone. Try a different power source and a different cable.

Probably, but first try a different cable and different USB power source.