Law 4 - Player's Equipment states
...Shorts - if undershorts are worn, they must be the same colour as the shorts ...
There's certainly nothing in the Laws of the Game and there don't seem to be any IFAB Additional Instructions regarding longjohns. I suppose you could argue that, as only undershorts are mentioned in the Laws, that's all you're allowed to wear? As far I can see, most people think that allowing a player to play in longjohns would be at the referees' discretion.
Also, from FIFA.com, "All items of jewellery are potentially dangerous. The term "dangerous" can sometimes be ambiguous and controversial, therefore in order to be uniform and consistent any kind of jewellery has to be forbidden.......Players are not allowed to use tape to cover jewellery. Taping jewellery is not adequate protection".
Strangely, The FA in England says that "Wedding rings must be rendered safe by taping to the satisfaction of the referee" 
Jag2009-07-21 13:52:05
Apparently I'm apathetic, but I couldn't care less.
"Being a Partick Thistle fan sets you apart. It means youre a free thinker. It also means your team has no money." Tim Luckhurst, The Independent, 4th December 2003