Usually you don't want to bias responses by making one sub-question stand out from the others, which is why via the question builder you cannot add any directives to change the font style or colour.
However, sometimes you just have to be able to do this. Using a custom template, and some CSS style sheet coding, you can modify many elements of your survey.
This is a task probably best left to those with some understanding of Cascading Style Sheets.
Watch the video for a walk-through:
HICS - Adding Background to Individual Answers from Hosted in Canada Surveys on Vimeo.