Are there common standards for aviation graphical user interfaces?
I think your best bet might be going through the Instrument Flying Handbook [PDF, Large!] by the Federal Aviation Administration. It has plenty of nice graphics, if you're fine with a lot of other irrelevant stuff in the document. There's also the Chapter 8, Instruments, of the FAA Pilot Flying Handbook [PDF], which is a bit more compact.





I'm not sure there are any exact specifications how these things have to look. I suspect the minimum requirements are given in the certification specifications, and how they are then presented may be up to the manufacturer. 

For instance, in EASA CS-23.2600B:


  The  applicant  must  install  flight,  navigation,  surveillance, 
  and powerplant installation controls  and displays  so  that  a 
  qualified  flight  crew  can  monitor  and  perform  defined  tasks 
  associated  with  the intended functions of systems and equipment. The
  system and equipment design must   minimise  flight crew errors, which
  could result in additional hazards


I'd interpret "minimise  flight crew errors" to not do anything overly ambitious with the instruments that deviate from the norm, i.e. what everybody else is doing.

In EASA CS-23.2610A:


  Each aeroplane must display in a conspicuous manner any placard and
  instrument marking necessary for operation.


It does happen that this stuff can be different- while the military is generally more flexible, the U-2 reconnaissance aircraft has a speed indicator that goes the opposite direction.