You will have to write a plugin. The logic should involve a layer to render the appropriate color as well as figuring out if the question has an answer that belongs to an expert user.
Regarding the rendering just overwrite the a_count() function to also include the qa-a-count-zero CSS class if the condition is met.
Regarding the condition I recommend to check the qa-include/pages/question.php file and see how the qa_db_full_child_posts_selectspec() function works. Then you'll have to call that function for every item in the question list and in the question itself to fetch all the answers of the question. Then it is just a matter of checking if the answer belongs to an expert user (QA_USER_LEVEL_EXPERT).