At this point, I'm assuming you're on a Mac. It may be possible to rig a keystroke up using OS or keyboard app/plugin, if it would be worth doing(?)
Rob, there is indeed a way to do it on a Mac
1) Open up your Mac's System Preferences
2) Navigate to Keyboard
3) Choose the Shortcuts tab
4) Click the "+" icon to add a shortcut.
5) In the Application: dropdown box, choose Adobe Photoshop Lightroom
6) For Menu Title use the exact word: Purple
7) Choose a keyboard shortcut. For me I've chosen F9 because it isn't used in Lightroom and it's right near "9" which is used for Blue.
That's all there is to it.
Here's a screenshot of what it looks like:
