Spark plug question

[Michel Bijl]

Folks,
Today I serviced the bike (oil\oil filter\air filter & spark plug change)
I noticed that the #1 spark plug had a nice dark grey color. Towards #4 the color 
becomes slightly darker on each spark plug. The # 4 plug is a very dark, leaning 
towards black color. Any ideas about this and should I worry about this ?
Thnx
Michel Bijl