Comment by adolph

The article links to a photographer of the phenomena which arguably has better info than the article.

https://paulmsmithphotography.com/pages/what-are-red-sprites...

  Sprites get their characteristic red color from excitation of nitrogen in 
  the low pressure environment of the upper mesosphere. At such low 
  pressures quenching by atomic oxygen is much faster than that of 
  nitrogen, allowing for nitrogen emissions to dominate despite no 
  difference in composition. As the atmospheric pressure increases in the 
  lower atmosphere, the red emissions are quenched and blue emissions from 
  atmospheric nitrogen excitation dominate. . .