I think the student was referring to the yellowish nighttime light you see in many parts of Tucson.

There are a lot of astronomical observatories in and around Tucson.  In an effort to reduce nighttime light pollution Tucson has enacted a fairly restrictive outdoor lighting code.  Outdoor lights must point downward and often sodium vapor lamps are used in street lighting or to light large areas like parking lights. 

The light from low pressure sodium lamps is monochromatic which means of one single color - yellow.  Here's a picture of  a low pressure sodium lamp (from Wikipedia)
And here's a graph of the spectrum of a low pressure sodium lamp



Lightning like this isn't as troublesome to astronomical observations because it can be filtered out.

High pressure sodium lamps are also used but they're not monochromatic.  Here's the spectrum of light produced by a high pressure sodium lamp.