If everything is working right what color should your spark plug spark be?

 

What part of the spark plug? There's more to reading a plug than just looking at the color of the insulator.

Start by reading all 5 of these pages.
http://www.centuryperformance.com/sp...ch-spg-26.html

Then read this page
http://www.empirenet.com/pkelley2/sparkplugreading.html

 

I didn't go read all that, but the short answer is: It depends. Differing amounts of ethanol in the gasoline makes an easily noticeable difference, but even which refinery it comes from can make a detectable difference.

 

If I am reading the OP correctly, he wants to know what color the spark should be, not the plug.

If so, a good rule of thumb is to make the spark jump a gap of about 1/4". This is easily done by inserting a phillips screwdriver into the end of the ignition wire in place of the spark plug, and then laying it down so that the shank is about 1/4" from a good ground. Obviously, you want to avoid doing this if you have raw fuel and/or vapors present.

Have a helper crank the engine (or use a remote starter switch) and the spark across that gap should be blue. If it is yellow, that indicates a weak spark. Trying the same thing with a spark plug may produce a blue spark, but the gap is smaller, so it's less conclusive. Plugs fire differently under compression.