Shortcut key to set color label on multiple pictures at once in lightroom CC 7.1?

If you forget what the shortcut keys are, they're always listed next to the commands on the menu. Choose Photo > Set Color Label and on the submenu, you'll see the 6, 7, 8, and 9 keys listed next to the first four colors.

If you are happy with the four first colours - you can use keys 6, 7, 8 and 9.

In Library, after activating Auto Sync when you make a new change such as this, it gets applied to all the other highlighted images - as well as, to the currently active image. This mode is powerful but potentially dangerous, so you may want to turn it off again afterwards...!