diff --git a/lib/Biodiverse/GUI/Tabs/Spatial.pm b/lib/Biodiverse/GUI/Tabs/Spatial.pm
index 260ac2f78..100a81051 100644
--- a/lib/Biodiverse/GUI/Tabs/Spatial.pm
+++ b/lib/Biodiverse/GUI/Tabs/Spatial.pm
@@ -379,6 +379,13 @@ sub get_tree_menu_items {
             },
             active   => 0,
         },
+        {
+            type     => 'Gtk2::MenuItem',
+            label    => 'Set colour for undefined list values',
+            tooltip  => 'Set the colour used to display list values that are undefined.',
+            event    => 'activate',
+            callback => \&on_tree_undef_colour_changed,
+        },
         {
             type  => 'submenu_radio_group',
             label => 'Colour mode',
@@ -589,6 +596,7 @@ sub init_dendrogram {
 
     #  cannot colour more than one in a phylogeny
     $self->{dendrogram}->set_num_clusters (1);
+    $self->set_dendrogram_colour_for_undef(COLOUR_GRAY);  #  default
 
     $self->{no_dendro_legend_for} = {
         map {$_ => 1, "<i>$_</i>" => 1}
@@ -821,6 +829,21 @@ sub init_dendrogram_legend {
     return;
 }
 
+sub set_dendrogram_colour_for_undef {
+    my ($self, $colour) = @_;
+    my $dendrogram = $self->{dendrogram};
+    return if !$dendrogram;
+    $dendrogram->get_legend->set_colour_for_undef($colour // COLOUR_GRAY);
+}
+
+sub get_dendrogram_colour_for_undef {
+    my $self = shift;
+    my $dendrogram = $self->{dendrogram};
+    return if !$dendrogram;
+    $dendrogram->get_legend->get_colour_for_undef;
+}
+
+
 sub init_grid {
     my $self = shift;
     my $frame   = $self->get_xmlpage_object('gridFrame');
@@ -1841,6 +1864,8 @@ sub colour_branches_on_dendrogram {
 
     my %done;
 
+    my $colour_for_undef = $legend->get_colour_for_undef // COLOUR_BLACK;
+
   LABEL:
     foreach my $label (keys %$listref) {
         next LABEL if $done{$label};
@@ -1864,7 +1889,7 @@ sub colour_branches_on_dendrogram {
             $colour_ref
                 = defined $val
                 ? $legend->$colour_method ($val, @minmax_args)
-                : COLOUR_BLACK;
+                : $colour_for_undef;
 
             $dendrogram->highlight_node ($node_ref, $colour_ref);
             $dendrogram->set_node_colour(
@@ -2623,6 +2648,29 @@ sub on_tree_colour_mode_changed {
     return;
 }
 
+sub on_tree_undef_colour_changed {
+    my ($self, $menu_item) = @_;
+
+    return if !$menu_item;
+
+    # Pop up dialog for choosing the hue to use in saturation mode
+    my $colour_dialog = Gtk2::ColorSelectionDialog->new('Select colour');
+    my $colour_select = $colour_dialog->get_color_selection();
+    if (my $current_colour = $self->get_dendrogram_colour_for_undef) {
+        $colour_select->set_current_color ($current_colour);
+    }
+    $colour_dialog->show_all();
+    my $response = $colour_dialog->run;
+    if ($response eq 'ok') {
+        my $hue = $colour_select->get_current_color();
+        $self->set_dendrogram_colour_for_undef ($hue);
+        $self->{dendrogram}->update_legend;
+    }
+    $colour_dialog->destroy();
+
+    return;
+}
+
 
 #  methods aren't inherited when called as GTK callbacks
 #  so we have to manually inherit them using SUPER::