diff --git a/src/Mod/Path/App/Voronoi.cpp b/src/Mod/Path/App/Voronoi.cpp
index abd4ca6450..f06e06e6d7 100644
--- a/src/Mod/Path/App/Voronoi.cpp
+++ b/src/Mod/Path/App/Voronoi.cpp
@@ -43,25 +43,6 @@ TYPESYSTEM_SOURCE(Path::Voronoi , Base::BaseClass);
 
 // Helpers
 
-static void colorExterior(const Voronoi::diagram_type::edge_type *edge, std::size_t colorValue) {
-  if (edge->color() == colorValue) {
-    // end recursion
-    return;
-  }
-  edge->color(colorValue);
-  edge->twin()->color(colorValue);
-  auto v = edge->vertex1();
-  if (v == NULL || !edge->is_primary()) {
-    return;
-  }
-  v->color(colorValue);
-  auto e = v->incident_edge();
-  do {
-    colorExterior(e, colorValue);
-    e = e->rot_next();
-  } while (e != v->incident_edge());
-}
-
 // Voronoi::diagram_type
 
 Voronoi::diagram_type::diagram_type()
@@ -208,10 +189,29 @@ void Voronoi::construct()
   vd->reIndex();
 }
 
+void Voronoi::colorExterior(const Voronoi::diagram_type::edge_type *edge, std::size_t colorValue) {
+  if (edge->color() == colorValue) {
+    // end recursion
+    return;
+  }
+  edge->color(colorValue);
+  edge->twin()->color(colorValue);
+  auto v = edge->vertex1();
+  if (v == NULL || !edge->is_primary()) {
+    return;
+  }
+  v->color(colorValue);
+  auto e = v->incident_edge();
+  do {
+    colorExterior(e, colorValue);
+    e = e->rot_next();
+  } while (e != v->incident_edge());
+}
+
 void Voronoi::colorExterior(Voronoi::color_type color) {
   for (diagram_type::const_edge_iterator it = vd->edges().begin(); it != vd->edges().end(); ++it) {
     if (!it->is_finite()) {
-      ::colorExterior(&(*it), color);
+      colorExterior(&(*it), color);
     }
   }
 }
diff --git a/src/Mod/Path/App/Voronoi.h b/src/Mod/Path/App/Voronoi.h
index 272a81321b..0634ef3c91 100644
--- a/src/Mod/Path/App/Voronoi.h
+++ b/src/Mod/Path/App/Voronoi.h
@@ -122,6 +122,8 @@ namespace Path
 
   private:
     Base::Reference<diagram_type> vd;
+    friend class VoronoiPy;
+    void colorExterior(const Voronoi::diagram_type::edge_type *edge, std::size_t colorValue);
   };
 
 } //namespace Path
diff --git a/src/Mod/Path/App/VoronoiPyImp.cpp b/src/Mod/Path/App/VoronoiPyImp.cpp
index ed3724483f..64b736b0ac 100644
--- a/src/Mod/Path/App/VoronoiPyImp.cpp
+++ b/src/Mod/Path/App/VoronoiPyImp.cpp
@@ -172,12 +172,45 @@ Py::List VoronoiPy::getCells(void) const {
   return list;
 }
 
+static bool callbackWithVertex(Voronoi::diagram_type *dia, PyObject *callback, const Voronoi::diagram_type::vertex_type *v, bool &isExterior) {
+  if (!isExterior) {
+    PyObject *vx = new VoronoiVertexPy(new VoronoiVertex(dia, v));
+    PyObject *arglist = Py_BuildValue("(O)", vx);
+    PyObject *result = PyEval_CallObject(callback, arglist);
+    Py_DECREF(arglist);
+    Py_DECREF(vx);
+    if (result == NULL) {
+      return false;
+    }
+    isExterior = result == Py_True;
+    Py_DECREF(result);
+  }
+  return true;
+}
+
 PyObject* VoronoiPy::colorExterior(PyObject *args) {
   Voronoi::color_type color = 0;
-  if (!PyArg_ParseTuple(args, "k", &color)) {
+  PyObject *callback = 0;
+  if (!PyArg_ParseTuple(args, "k|O", &color, &callback)) {
     throw  Py::RuntimeError("colorExterior requires an integer (color) argument");
   }
-  getVoronoiPtr()->colorExterior(color);
+  Voronoi *vo = getVoronoiPtr();
+  vo->colorExterior(color);
+  if (callback) {
+    for (auto e = vo->vd->edges().begin(); e != vo->vd->edges().end(); ++e) {
+      if (e->is_finite() && e->color() == 0) {
+        const Voronoi::diagram_type::vertex_type *v0 = e->vertex0();
+        const Voronoi::diagram_type::vertex_type *v1 = e->vertex1();
+        bool isExterior = false;
+        if (!callbackWithVertex(vo->vd, callback, v0, isExterior) || !callbackWithVertex(vo->vd, callback, v1, isExterior)) {
+          return NULL;
+        }
+        if (isExterior) {
+          vo->colorExterior(&(*e), color);
+        }
+      }
+    }
+  }
 
   Py_INCREF(Py_None);
   return Py_None;