Hello and welcome to the forum 
First of all, please reconsider writing Sketch plugins in 2025. The whole plugin infrastructure is barely supported at this point, and Sketch folks made it clear that this won’t change anytime soon.
As someone who’s been building automation tools for Sketch practically full-time since 2016, the best advice I can give you is to spend your time and energy on something else, rather than fighting this never-ending uphill battle that is making a non-trivial Sketch plugin.
This advice comes from a place of huge love, not hate 
Anyways, in order to deal with imported/foreign stuff you’d definitely need to drop to the native API layer.
Let’s say we have a Rectangle that references 2 color variables:
My Pink is a document-local variable used as a border colorApple iOS UI / System Colors / systemGreen – Dark is a variable we’ve imported from the “Apple iOS UI” library, used as a fill color
Now, let’s start by taking this selected Rectangle and extracting raw native color objects from its fill and border:
const Document = require('sketch/dom').Document
const Swatch = require('sketch/dom').Swatch
const { toArray } = require('util');
const document = Document.getSelectedDocument()
const layer = document.selectedLayers.layers[0]
// These are native MSColor objects
const borderColor = layer.style.borders[0].sketchObject.color()
const fillColor = layer.style.fills[0].sketchObject.color()
The next step is to use find out if those native color objects have a reference to a swatch:
function swatchForMSColor(nativeColor) {
const documentData = nativeColor.documentData()
const swatchID = nativeColor.swatchID()
if (!swatchID) { return null }
return Swatch.from(documentData.swatchWithID(swatchID))
}
const borderColorSwatch = swatchForMSColor(borderColor)
const fillColorSwatch = swatchForMSColor(fillColor)
These swatches are some actual Sketch JS API objects so they print nicely and everything. The problem is that we don’t know which one is local and which one is imported from a library (and which library?). Let’s refactor swatchForMSColor() a bit and inject a few useful helper methods into the resulting swatch object:
Disclaimer: I’ve no idea how to write JavaScript when it comes to objects and classes. The code below works, but there’s probably a more idiomatic way to do it.
function swatchForMSColor(nativeColor) {
const documentData = nativeColor.documentData()
const swatchID = nativeColor.swatchID()
if (!swatchID) { return null }
let swatch = Swatch.from(documentData.swatchWithID(swatchID))
// Returns an actual foreign swatch object if any
swatch.foreignReference = function () {
const nativeObject = this.sketchObject
const allForeignSwatches = toArray(nativeObject.documentData().foreignSwatches()) || []
return allForeignSwatches.find((s) => s.localSwatch().objectID().isEqualToString(nativeObject.objectID()))
}
// Returns `true` if this swatch comes from a library
swatch.isImported = function() {
return !!this.foreignReference();
}
// Returns a source library identifier for this swatch if one is available
swatch.libraryID = function() {
if (!this.isImported()) return null;
return this.foreignReference().libraryID()
}
// Returns a source library name for this swatch if one is available
swatch.libraryName = function() {
if (!this.isImported()) return null;
return this.foreignReference().sourceLibraryName()
}
return swatch
}
With that new code in place, let’s see which of those swatches is which:
console.log(`Is border color a variable? ${!!borderColorSwatch}`)
console.log(` The variable name is "${borderColorSwatch.name}"`)
console.log(`Is border color a *library* variable? ${borderColorSwatch.isImported()}`)
// Is border color a variable? true
// The variable name is "My Pink"
// Is border color a *library* variable? false
console.log(`Is fill color a variable? ${!!fillColorSwatch}`)
console.log(`Is fill color a *library* variable? ${fillColorSwatch.isImported()}`)
console.log(`Fill color variable "${fillColorSwatch.name}" comes from library "${fillColorSwatch.libraryName()}" (${fillColorSwatch.libraryID()})`)
// Is fill color a variable? true
// Is fill color a *library* variable? true
// The color variable "System Colors/systemGreen - Dark" comes from library
// "Apple iOS UI" (DD59C2AC-DA86-465C-B5D7-FC1979E0DDB8)
Hope it helps!
Thank you!!!
