Spark plug color chart(thanks to Saber6)

[mud99]

How to Read Spark Plug Colors (written by Atlantic Jet Sports)
In General: Reading spark plugs can be a valuable tuning aid. By examining the spark plug insulator color, an experienced personal watercraft engine tuner can determine valuable information about the engine's overall operating condition.

Normal: Grey to Light Golden-Brown Color

This condition is ideal, the spark plug and engine air/fuel mixture are operating properly.
Dry: Black Soot Buildup

Air/fuel mixture is too rich, the carburetor settings are incorrect, or the flame arrestor is dirty or has mounting problems.
Spark plug heat range is too cold for the operating conditions.
Ignition system problems causing a weak or intermittent spark.
Wet Fouling: Shiny, Wet, Black Appearance

Excessive use of the choke (gas fouled)
Prolonged low rpm operation (gas or oil fouled)
Fuel to oil ratio is too rich (oil fouled)
Excess Deposits: Bumpy, Chalky Buildup

Poor fuel quality
Oil leakage into combustion chamber
Improper oil used for premix/injected
Overheated: White, Blistered, Melted Electrode

Lean air/fuel mixture
Spark plug heat range is too hot for operating condition of the engine
Plug is not properly gapped and/or torqued onto head
Overly advanced timing
A detonation problem would show signs, such as silver specs, black specs, or melting or breakage at the firing tip.

NOTE: Signs of fouling or excessive heat must be traced quickly to prevent further deterioration of performance and to prevent possible engine damage.