I tried a bit and it seems i get trouble first time I query one property: $lb->itemcget($sel, '-background') so I provide the initial value manually: my $current_color_bg = $lb->itemcget($sel, '-background') || 'yellow';

Here a working example flipping the colors:

use strict;
use warnings;
use Tk;
my $mw = MainWindow->new;

my %next_color=(red=>'yellow',yellow=>'red');

my $lb = $mw->Scrolled(
    "Listbox",
    -bg        => 'yellow',
    -fg        => 'red',
     -scrollbars    => 'e',    
)->pack;
$lb->insert('end', "Item1", "Item2", "Item3");

$lb->bind(    '<Double-1>',
    sub{
        my $sel =  $lb->curselection()->[0];
        print "pressed $sel\n";
        my $current_color_bg = $lb->itemcget($sel, '-background') || '
+yellow';
        my $current_color_fg = $lb->itemcget($sel, '-foreground') || '
+red';
        print "current color of the element $sel is $current_color_bg 
+on $current_color_fg\n";
        $lb->itemconfigure( $sel, 
                            -background=> $next_color{$current_color_b
+g},
                            -fg=>$next_color{$current_color_fg}
        );    
    }
 );
MainLoop;
[download]

Double clicking the first entry 3 time and one time the second:

pressed 0
current color of the element 0 is yellow on red
pressed 0
current color of the element 0 is red on yellow
pressed 0
current color of the element 0 is yellow on red
pressed 1
current color of the element 1 is yellow on red
[download]

L*