Not all tachometers are the same. It might be helpfull to know what brand you have, then someone might have the installation instructions for that particular unit.

Some thoughts. First, unlike house wiring, green is not necessarily ground. More often, while not always, in automotive it is the black wire. Likewise, the white MAY be for an internal light. Look around the housing to see if there is a "switch" of some kind for selecting the number of cylinders. If you have dual points typically (meaning not always) you would chose 4 cyl for an 8 cyl engine. Look at where the wires attach inside the housing (if there is one). Hopefully one of the connections is marked "S", that would be the one that goes to the "-" on the coil. The Batt or "B" terminal would go to the "hot" side of the ignition switch, or a switched source of power. Sometimes the lead from the coil will require a filter to inhibit cross-over signal interference. Again, that would depend on manufacturer. Hopefully it's not one of the old style tachs that require a remote sending unit and you only have half of the system.