When I read the 2013 AR and DA PAM 670-1 , it stated Service Members must wear dull color underwear in duty uniform. Who checks that?

I don't care who you are or what your rank is, if you come to me wanting to check my underwear, you're going to get your feelings hurt. If you press the issue, you better have MPs backing you up. This is beyond ridiculous.

Wear underwear?!?! Haven't done that for 15 years! #Commando

In the 90s, the biggest swinging dick in the USAF made a rule that pilots were required to wear V-neck tee shirts when they wore the flight suit. Gotta have that manly chest hair visible to prove you're a warrior, you know.

Mickey Mouse rules have two real reasons for existence in the military.
1) They keep us accustomed to doing things that make no sense. Why is that needed, you ask? Because war often makes no sense. In the moment when stress and fear would make you want to question every order to move forward, leaders must know that their people will accept and execute orders, even if they make no sense to the individual. So we practice "following orders that make no sense" every day.
2) Sometimes stupid shit gets past the commander and his staff, and he signs it into regulation.