To colour-code a new extension ( .ctp CakePHP templates for example ), edit the file C:\Program Files\Adobe\Adobe Dreamweaver CS3\configuration\Extensions.txt and add the .ctp extension to the first line ( All Documents ) and the PHP Files section :
PHP,PHP3,PHP4,PHP5,TPL,CTP:PHP Files
Next, edit C:\Program Files\Adobe\Adobe Dreamweaver CS3\configuration\DocumentTypes\MMDocumentTypeDeclarations.xml and add the .ctp extension to the appropriate section - in this case :
winfileextension="php,php3,php4,php5,ctp" macfileextension="php,php3,php4,php5,ctp"
Restart dreamweaver and you should be good to go. The paths will be a little different in CS2 and below, probably under C:\Program Files\Macromedia\Dreamweaver