NetBeans handles nearly all file types and colour codes it all very nicely. However it doesn't recognise Smarty template files (.tpl) and finding where I can add .tpl files to NetBeans always baffles me.

Tools >> Options >> Miscellaneous >> Files

Then click 'New' next to the 'File Extension' dropdown and enter 'tpl'. Associating it with the HTML file type worked better for me than PHP.