Does the "Disease Zones Expand" emergency event only apply to cities with cubes of more than one color?
In this thread on BGG, game designer Tom Lehmann addresses this concern. User joekeck posted this as a reword:


  "if any cities contain any cube(s) (other than purple) that do not match that city's color, then add an additional cube of that cube color"


and Tom responded:


  In spirit, yes. The problems with this wording (which is similar to the wording I originally submitted to Z-Man) are: A) "that cube color" could conceivably be read as a cube of the city's color ("that" isn't unambiguous here) and B) it's too long for the card text area.


So in your example, both Los Angeles and Mexico city receive an additional blue cube, but no yellow cubes are added.