Purple is 12V constant and Black* is Ground on pins 1 & 2 respectively on both Speedo & Tach connectors
Ignition Switched power can be found on Green/Red on pins 3 & 4 of each connector
* Be careful with the Black - there are multiple Black wires on each connector and not all are ground!
Be sure to use the one on Pin 2 (between the Purple and Green/Red wires)
The final caution is with regard to what you are connecting there: just be sure not to exceed the current rating of wires and fuses.
What exactly are you adding?
A better source might be one of the auxiliary connectors, which have constant power, switched power and ground already there - should be in the area behind the steering head.