Purple => IGN
Black => GND
Gray => SND (it might be "grey", in jurisdictions where "color" is spelled "colour")
gauge illumination (tie into another gauge lighting wire) => LT
If you have the OMC control box, there's a socket covered by a rubber boot, located just under the throttle friction control knob (which is just under the keyswitch), that accepts a plug&play tach wiring harness.