When you use want to replace this code

$page = $this->newPage();

by this:

$pdf->pages[] = $pdf->newPage('4:6:'); 

but the results is:

Fatal error: Class 'Zend_Pdf_Color_RGB' not found app/code/core/Mage/Sales/Model/Order/Pdf/Shipment.php

It is common issue, if you are using Windows you can't see this issue, but when you upload it to your Linux server you will see.

It caused by Windows doesn't care about the uppercase, but Linux cares about it.

Actually there is no class Zend_Pdf_Color_RGB. Its Zend_Pdf_Color_Rgb. Check your code and replace Zend_Pdf_Color_RGB with Zend_Pdf_Color_Rgb