'cause no one's searching for them, it's tag bloat. More pointless tags on an image that can be better spent describing the type of weapon or something else far more critical about the post than the colour of the weapon. Weapons may be an integral or focal part of a design or image but that doesn't justify abusing them with colour tags that no one is going to be looking for. The search statistics for those will be on the floor and then we'll see a nuke BUR in a few months.
A tag to describe a cutesy or unique type of weapon - which in this case came out to be pink weapon - makes a bit more sense because of how common pink is used as a weapon colour to portray cuteness. Anything beyond that is beyond dumb to taggers and searchers.