New config field extruder_color for preview of extruder assignment.

lib/Slic3r/GUI/OptionsGroup/Field.pm

@@ -423,8 +423,10 @@ sub get_value {
 
 sub _string_to_colour {
     my ($self, $string) = @_;
-    
+
     $string =~ s/^#//;
+    # If the color is in an invalid format, set it to white.
+    $string = 'FFFFFF' if ($string !~ m/^[[:xdigit:]]{6}/);
     return Wx::Colour->new(unpack 'C*', pack 'H*', $string);
 }