Wrong forum, dood! Post this in the right place next time.
Assuming they really were all changed at the same time (no guarentee) looks like there's a bit of excessive crap on those two.
One of two things: Either they are worn out or it's carbon deposits. However like RGal said, carbon deposits tend to be gray-black, not light. I do see hints of dark stuff on either side of the air gaps, though that may be due to lighting.
Cold Fouling AKA cabon-fouled is caused by an excessively rich air/fuel mixture. If only one or two plugs show evidence of it, sticking valves are the likely cause.