What is the ruling on the exposure of the awrah due to: not fully zipping up one’s trousers, the color of the skin being visible through thin clothing, or the awrah being visible from above or the side through buttoned shirts?

A Muslim must cover his awrah (private parts), and this is a matter of scholarly consensus. He should strive to cover them without being overly strict or falling into waswasah (whispers/obsessive doubts), because Satan approaches a person through exaggerated acts of obedience to cause him to fall into waswasah and weariness.

If there is an opening in the trousers that clearly exposes the awrah, and it is not merely waswasah, then it must be covered by wearing underwear or by pulling the shirt over the trousers. As for shirts with buttons, there is no harm in them if the buttons are not in the place of the awrah; otherwise, one must wear something underneath that covers the awrah.