Could you please look at my spark plugs?

[jsap]

I just pulled my plugs on my 97 SL500 for the first time, and I don't like what I see. The car's been running fine, though I've been ignoring some minor uneven idling. But now that I see the plugs, I can see why.
Also, though it doesn't show in these pics, the center electrodes on the 2 plugs I've pulled look like melting ice-cream cones. On both, the electrodes have worn down, much more so toward the ground electrode direction. (It's like a slope going down toward the ground electrode side.)

Plus, the back side of the ground electrode looks powdery, and it's yellow-greenish.

The wet carbon fouling could be because I've been running 91/87 fuel mixture the past two tanks (before that I've been strictly 91), because the ceramic is still fairly clean. But the overall plug is rusty, and I definitely need new plugs.

Could anyone provide some insigtful diagnosis