I got this in PoPo City at Betty's Gum Store.
So to get this achievement you need to buy the colour mods for both the bullets AND the gun itself. They cost 22 smurt each.
Once purchased you need to select the colour mod and put it in the machine to the right of Betty (change what you are interacting with
), then select the desired weapon and put that in the machine. Once they are both in the modifier will be applied.
Do this to both the bullets and the gun and the achievememnt will pop.