could mean one of the following:
worn: rounded electrodes, reddish color... get new ones.
high speed glazing: yellowish glazed appearance, combustion chamber temp has risen suddenly during acceleration (misfires at high speeds)... get new ones, and the correct size.
normal: brown, to tan color... alright.
this is straight outta the back of the Haynes manual for our car, look at pep boys or any other place like that...