diff -r 283aa6225d98 -r d43434c60d3a src/Tools/Graphview/visualizer.scala
--- a/src/Tools/Graphview/visualizer.scala	Mon Jan 19 16:31:04 2015 +0100
+++ b/src/Tools/Graphview/visualizer.scala	Mon Jan 19 16:38:01 2015 +0100
@@ -168,5 +168,7 @@
     else
       Node_Color(foreground_color, model.colors.getOrElse(node, background_color), foreground_color)
 
-  def edge_color(edge: Graph_Display.Edge): Color = foreground_color
+  def edge_color(edge: Graph_Display.Edge, downwards: Boolean): Color =
+    if (downwards || show_arrow_heads) foreground_color
+    else error_color
 }
\ No newline at end of file