Boxerworks.com

You may have a bad coil. OR a bad wire or a bad SP cap.
Are you still running the original coils? and wires?
Are you still using the original points/capacitor? (If electronic, do not test for spark by pulling a lead, but use another plug connected to that lead and laid on top of the engine to watch for spark!! Saves your electronic goodie.)
So...IF your coils are original, one may not be working at speed, in that it has a fault inside that allows a spark at lower rpm but as the frequency increases, it cannot operate properly.
Been there, Done that.
NEW coils fixed a LOT of problems. Milage included!

IF you decide to go for new coils and get the BOSCH "BLUE" (which aren't Blue anymore, I see), BE SURE TO CHECK for diameter clearance as I've heard the new ones are fatter and a bit shorter than the old ones. They CAN rub on the underside of the gas tank if you aren't careful when you install them.

As Vanzen said, fuel doesn't color plugs the way it used to, so the lighter one may be perfect, while the one with color may have had a bit more oil in the cylinder when you started up.

IOW"s, if it's running well, don't sweat it.

True about color, but you CAN tell if a cyl. is running rich as that plug will be darker than the other. If your choke is not being shut off, that side will be definitely black.
Anyhow, MS, I understood from the first post that it wasn't running "well", at least not well enough to suit.
It took me quite a while to finally recognize my coil problem. Nothing (electrical-testing) showed up when they were cold and not under "working stress".