When using the phonetic alphabet, for airways, should we say Uniform or Upper?
ICAO

ICAO Annex 11 and Doc 4444 confirm the usage of Upper.

Airways are four groups per ICAO, the older group is A, B, G, and R. Those stand for Amber, Blue, Green, and Red. There are three other groups.

So an airway named UG1 will be called Upper Green (not Golf) One on the radio.

Example they give in Doc 4444:


  N0450F310 L9 UL9 STU285036/M082F310 UL9 LIMRI
  
  (...) the flight will proceed on Airways
  Lima 9 and Upper Lima 9 to a point bearing 285 degrees magnetic and 36 NM from the Strumble VOR. From this (...)




Jeppesen Airway Manual also confirms it, check page 251:





FAA (for airways only)

Q is called Q (kyo͞o) on the radio (not Quebec), J is called J, not Juliet or Jet (Jet is what it means). See FAA JO 7110.65X 2−5−1 AIR TRAFFIC SERVICE (ATS) ROUTES.


  VOR/VORTAC/TACAN airways or jet routes.
  State the word “Victor” or the letter “J” followed by
  the number of the airway or route in group form.
  
  EXAMPLE−
  “Victor Twelve.”
  “J Five Thirty−Three.”


The FAA is similar in using the color code for the aforementioned group:


  Colored/L/MF airways. State the color of the
  airway followed by the number in group form.
  
  EXAMPLE−
  “Blue Eighty−One.”


Also check the FAA Instrument Procedures Handbook, page 2-24 of Chapter 2, here are two examples they give:


  UR5—Upper Romeo Five
  
  UW456—Upper Whiskey Four Fifty Six


Yes the FAA handbook does not agree with their ATC Job Order regarding R being Red (this will be for the old airway system such as those still in Alaska).



Note: the question asks about airways, the information here is not for tail numbers, navaids, etc., only airways.

See here for how the FAA decides on the designations: How are Victor airways defined and numbered?