$ perl -le'
    $mac = "0:13:46:b:4:31";
    $mac =~ s/([^:]+)/ sprintf "%02x", hex $1 /eg;
    print $mac;
'
00:13:46:0b:04:31

$ perl -le'
    $mac = "0:13:46:b:4:31";
    $mac = join ":", map sprintf( "%02x", hex ), split /:/, $mac;
    print $mac;
'
00:13:46:0b:04:31