Would mostly rely on using factory recommended sparkplug (B8ES) for most of us under normal riding conditions using the full rpm range in a mix of city and highway riding.
Hotter sparkplug (B7ES) runs hotter to help keep plug cleaner and avoid carbon loading under conditions of sustained lower rpm usage such as plonking around town all the time without some regular higher rpm usage (such as an occasional hard acceleration rip through the gears to "blow it out.") The original Z1 Owners Manual said B7ES could be used during the break-in period if B8ES plugs were fouling, but cautioned in bold print to use B8ES after break-in because continued use of B7ES for hard acceleration or high speed could cause engine damage from overheating. The FSM warns further about preignition and burning holes in pistons.
Colder sparkplug (B9ES) runs cooler at sustained hard high rpm usage such as extended high speed operation or road racing to help hold the temp about where it would be with a B8ES under more normal operating conditions. The original Z1 Owners Manual said B9ES could be used for cooler running if ceramic on B8ES plugs was burning white. The FSM warns further about several additional problems caused by carbon build-up on the colder plugs.
Where non-defective properly gapped B8ES plug readings are too dark or too light, it's almost never the plug's fault. But more likely a problem with the ignition, timing, compression, or carbs.
Good Luck!