Just specify RGB in the format of '#rrggbb', for example, yellow would be '#FFFF00'.

use Tk;
use Tk::Tree;
use Tk::ItemStyle;

use warnings;
use strict;

my $main = MainWindow->new (-title => "Test tree");
my $tree = $main->ScrlTree(-itemtype => 'text', -separator  => '/', -s
+crollbars => "osoe");
my $yellow_style = $tree->ItemStyle('text', -refwindow => $tree, -bg =
+> '#FFFF00');
my $green_style = $tree->ItemStyle('text', -refwindow => $tree, -bg =>
+ 'green');
$tree->add(1, -text => 'abcd', -itemtype => 'text', -style => $yellow_
+style);
$tree->add(2, -text => '1234', -itemtype => 'text', -style => $green_s
+tyle);
$tree->autosetmode;
$tree->close (1);
$tree->pack(-fill=>'both',-expand => 1);

MainLoop;
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