Several shades of spark
There's been some recent chat about judging the effectiveness of the spark across the plug electrodes by its colour. Way back in my greasy fingered career, I remember being told "it's not the colour that matters but the intensity". So rather than express a personal opinion with the risk of heated discussion, I've done a bit of digging and found the following buried deep in a 1998 Briggs and Stratton publication. I have abridged it slightly but the subject matter is their's, not mine.
To dispel a commonly held belief, the spark on an engine is satisfactory if it jumps the 0.166" gap regardless of colour.The spark colour is more the result of prevailing atmospheric conditions, such as humidity. A good spark can be fat blue one day and thin yellow the next day according to the dry or moist air conditions prevailing on that day. Many mechanics look only for a fat blue spark, which is a fallacy. A red, yellow or any other colour is fine by the engine so long as it jumps the correct tester gap.
In my toolbox is this tester for the checking sparks ability to jump the prescribed gap.
I have the commonly used type that looks like a spark plug with a central electrode and spot welded on crocodile clip, gap looks like about three eighths of an inch though, maybe enough to cause a breakdown in an old coils wiring!
I'm not getting into any heated discussions, but I know what I prefer to see and leave it at that.