I think a nice way to do it would be to just use the longtitle field, but have the user wrap the coloured part in asterisks (*). Then an output modifier would replace them with <span> tags when inserting into your page.
Create a snippet called
spanwrap or something that makes more sense with the following (not tested):
<?php
return preg_replace('/*(.*?)*/', '<span>\\1</span>', $input, 1);
Then in your template add the modifier to the longtitle tag:
<h2>[[*longtitle:spanwrap]]</h2>
And get your users to enter the longtitle text like this:
Longtitle: This is my awesome *headline*
☆ A M B ☆