I just change the spark plugs for my friend, he drives a LS notec 95 integra(60Kmiles, IHE), i found the spark plug on the is rusted out(brownish) what's going on?? is it normal to have all three plugs in good condition and the other rusted out?

 

is it rusted or just rust color, i believe that a rust color is normal, but it its rusted... i have no idea