#!/usr/bin/perl

# http://perlmonks.org/?node_id=1171182

use strict;
use warnings;

$_ = join '', <DATA>;

1 while s/^(\S+ \S+ ).*\K\n((?:.*\n)*)\1(.*\n)/:$3$2/m;

print;

__DATA__
A1 abc yellow
B1 xyz green
A2 cde red
A1 abc green
A2 cde yellow
A1 abc blue