3 Plugs Ok-1 Plug??????

[dsm5150]

i pulled my plugs today when i got to work(bored)any way one plug has a rather white spot on it.the others have a nice even tan/brown burn color to them.im running 15# on a big16,190lph pump.what gives?is this major?what can i do to fix it?
i have searched but have yet to see a good link on the right way to read plugs.

 

[mitsuspeed]

acording to the little chart in the back of the haynes manuel:

TOO HOT

symptoms:blistered white insulator, eroded electrode and an absence of deposits.

recomendation:check for corect plug heat range, over -advanced ignition timing, lean fuel mixture, intake manifold vacume leaks, sticking valves and insufcient enigne cooling