How to add a colon for Webform field labels?
With D7 webforms, you should be able to access the parts of the webform (rendered within webform-form.tpl.php) within a form alter:

function mymodule_form_alter( &amp;$form, &amp;$form_state, $form_id ) {}


If you have the devel module enabled, you can use it to print out the variables within the $form['submitted'] array - for example, to edit the title of a form component, you would use:

$form['submitted']['my_form_component']['#title'] = 'My new title';


To add a colon to the end of each component's attribute (i.e. to add a colon at the end of the #title attribute) you could set 

$form['submitted']['my_form_component']['#title'] = $form['submitted']['my_form_component']['#title'] . ":";


If you use a variable instead of the specific 'my_form_component' name, you can set the form_alter function to loop over and add the ":" to the end of all component titles.

Small example here:
https://www.drupal.org/node/1558246