Neither is good. But rich if it has to be off
Leaner is faster and more powerful, but will run hot and run you the risk of overheating, pre ignition, detonation, blown piston/rings etc. Richer on the other hand will last longer but will not have a much power and you may foul plugs and use more fuel etc.

Personally on my own machines I run them perfect or maybe slightly rich (dark tan) just for a long engine life.
Basically what I do is start out rich on the main jet and work my way down one jet at a time until it runs great and has an ok color (brown or dark tan) then I go another size leaner.

For race engines I will keep going leaner until the plug is a grayish/very light tan.

 

Go here and look at the picture. This is for two strokes but it should be close to the same.
http://www.pilot-odyssey.com/~hoser/web/sparkplug.htm

 

Definitly Tan

 

Hey Wanabe1, what kind of quad do you have?

 

A, your going to have to tell me what gas you run before I can answer your question. Race gas? Pump gas? If it looks that dark after that run, I would say your rich, to way rich.

 

depend's on the fuel.