diff -ru gcolor2-0.4.orig/src/callbacks.c gcolor2-0.4/src/callbacks.c
--- gcolor2-0.4.orig/src/callbacks.c	2005-07-12 14:06:12.000000000 -0400
+++ gcolor2-0.4/src/callbacks.c	2020-04-27 10:06:57.988405153 -0400
@@ -12,6 +12,9 @@
 #include <string.h>
 #include <glib/gprintf.h>
 
+GtkWidget        *gcolor2;
+GtkWidget        *menu;
+GdkColor          colorvalue;
 GtkWidget        *savedialog;
 gchar            *colorname;
 static GtkWidget *aboutdialog = NULL;
diff -ru gcolor2-0.4.orig/src/callbacks.h gcolor2-0.4/src/callbacks.h
--- gcolor2-0.4.orig/src/callbacks.h	2005-07-11 19:18:59.000000000 -0400
+++ gcolor2-0.4/src/callbacks.h	2020-04-27 10:05:55.157028236 -0400
@@ -41,9 +41,9 @@
                            "\302\267 Usability / user interface enhancements\n" \
                            "\302\267 Code cleanup"
 
-GtkWidget *gcolor2;
-GtkWidget *menu;
-GdkColor   colorvalue;
+extern GtkWidget *gcolor2;
+extern GtkWidget *menu;
+extern GdkColor   colorvalue;
 
 enum
 {