I've read that Briggs article before. Personally I think it is only partly correct. Yes, the nature of the gas in the spark gap can change the spark colour. However the spark colour does indicate the temperature of the gas in the spark gap. Observing the colour is a standard way of measuring that temperature (known as the "colour temperature", because optical instruments are the most commonly used and convenient way to measure it). So, a blue spark is a hot one, and a red spark is a cool one. An orange spark in my experience so far is always an inferior one, but I believe Briggs when they say that having the wrong gas in the spark gap can reduce the spark temperature.
