Avbug had the right answer. I found a reference in the NACO Aeronautical Chart Users Guide. It refers to large cities and towns and is based on population density. Nothing to do with lights but I suppose many homes have electricity these days
If its yellow, it means a populated area and you have to maintain 1000 feet above the highest object within a 2000 foot radius of it. If its not yellow, than you only have to maintain the 500 feet from any vessel, structure, etc.
At least that it how it used to be. The DE I took my commercial with told me that there was a court case where two people standing next to each other qualified as a "densely populated area", if you believe that. Either way, don't go vuzzing anything regardless of what color it is on a sectional or you might get a visit from one of our friends at the FAA.
The depictions of populated areas on aeronautical charts have never been intended to define minimum altitudes with respect to 14 CFR 91.119. Never rely on depictions on an aeronautical chart to justify your separation from objects, obstacles, or persons.
91.119 does not refer to densly populated areas, but congested areas. It also refers to sparsely populated areas. Formerly, the regulation spoke to built-up areas. Specifically, today, 91.119 identifies congested areas as cities, a town, a settlement, or any open air assembly of persons. Yes, one or two houses constitutes a settlement and qualifies for increased separation, as does a gathering of two or more persons.
Aeronautical planners make a general attempt to roughly define the shape of the city when depicting large populated areas on aeronautical charts. Because these areas are lighted at night, this will generally conform to the basic shape or outline of the populated area. However, no attempt is made to be exact, or specific, and one should not expect to see the same thing on the ground that one sees on the chart. Population centers change, power outages occur, etc.
The yellow on the chart is strictly to show a population base as a reference. It shouldn't be used, or implied in use, in any other context.
I attended a safety seminar that had as it's guest speaker the guy who, with his team, flies around the country updating and making changes to the charts. He works for the NOA, I believe. His answer?
-it is meant to give a visual cue as to the general shape of a populated area, as seen from the air, AT NIGHT
he went on to say that these can certainly change and that all pilots are encouraged to look for things that are lacking and contact him/the agency with that info