Valve lash settings would be low on the list of things to look at especially with no abnormal valve lash noises.  A worn distributor can have the timing variable thoughout the engine so that is worth considering.  Degreeing in the damper at 90° intervals and checking the timing on each cylinder would be the easiest way to check this without pulling the distributor and putting it on a distributor machine.

But looking for a vacuum leak would be high on the list depending upon what’s causing the other spark plugs to be darker.  A vacuum reading off of the intake or from below the carburetor might give a idea of where to look.  A cranking compression test can also give a clue as a significantly higher cranking compression reading can contribute to a cleaner or leaner spark plug reading.  Looking for commonalities on the cylinders that have similar appearances can also point you towards either vacuum leaks or carburetor related issues.  For example, white spark plugs on cylinders 1 & 2 would point to a vacuum leak while white spark plugs on cylinders 1 & 4 would point to a carburetor issue.  If using the ECZ-B intake, then lean plugs on cylinders 7 & 8 would have you looking for a vacuum leak coming from one of the vacuum connections at the back of the intake manifold.  The list of combinations is numerous.