There’s seems to be no way of adding limited edition versions of instruments if the base model already exists right ?
Example
Good question. Whether or not the base model is already in the system has nothing to do with it.
The determining factor, at the moment, seems to be whether or not Reverb chooses to grant that special edition a canonical landing page.
Canonical, in this case, means that the there is a page for the item like this with a dedicated price guide, details page, etc.
I'm still learning about how/when Reverb grants these pages to certain models, but there's not a ton of consistency to it. This pearl white limited edition model could have easily had its own landing page on Reverb... it just doesn't, for whatever reason.
Even though Reverb doesn't own Equipboard, you can think of it like Disney deciding what is Canon for all the Lucasfilm/Star Wars stuff. Fans may love a character from some 90s Shadows of the Empire novel, but if Mickey says it's not canon right now, it's not canon.
The Admins want us to stay in-sync with Reverb item taxonomy, because once an item is no longer in production, Reverb is likely the only place where Equipboard can make any affiliate dollars off of linking to that item... and affiliate dollars are one of the very few ways this site can pay the bills. This isn't the only reason for staying in-sync with Reverb, but it's A reason. Maybe it's just a fan theory...
In this example you gave, if you're logging a Monologue XD for an artist, both the XD and the pearl white limited edition sound exactly the same, so there isn't much to gain from adding the limited edition in that context.
If someone wanted to add the white XD because they own the white one and they want their own Equipboard to reflect that, that's 100% understandable.. the Admins are working their way towards being able to somehow show that you own a specific color of a guitar or synth, but they're just not all the way there yet.