On the actual prop, the light on the back knob shows what the phaser is set to: blue for stun or red for kill.  With these pieces, you can swap out different color back knobs so your phaser can be "set" how you like it.

And why limit yourself to just blue and red?  Print purple for phaser slap that makes everyone say, "OH!  That's going to leave a mark!"  Or maybe you worked hard to wear command gold, and so it's only fair that your hard working phaser wears gold too.  Print whatever colors you want.  Now your phaser can coordinate with your outfit and/or your mood.  And admit it, secretly you've always wanted to say, "Set phasers to FUN, Mr. Sulu!"  Swap in a bright green knob, and you're ready for St. Patrick's Day.

I hollowed out the power pack and made a removable lid so you can store three extra knobs in the power pack.  The knobs stay attached to the phaser by 6x2mm magnets.  This base of this knob is 1mm smaller than the original so it will fit into power pack, and the light is 2mm higher so it's more visible.  So you do need to print these versions even though they look almost identical to the versions of the original model. 

And if anyone want to mod my mods, my Tinkercad files are here: Star Trek SNW Phaser Back Knob - Tinkercad