Change the color of a group (inside an index)

Good evening to everyone,
I'm creating questionnaires that have very long and complex questions (divided into groups).

The questionnaire displays the group index to allow the user to move from group to group faster.

Since the questionnaire is nominative, and it is very probable that the user completes it several times, my client asked me if it is possible to report to the user the sections already compiled, perhaps changing the color of the group within the index .

Does anyone know if there is a control or a parameter that I can query to understand / set the group's completion status?

The only method that came to my mind is a sort of mega equation of relevance but I'm not even sure it will work, so I wanted to ask first.

The version of Limesurvey in use is this: 2.05+ Build 141229, but in any case it is also good to accept recommendations for later versions (3.x)

Thank you all

Thank you very much!
It works!

I've added this single row:

#index .missing {color:/*your preferred color */; }

and now the questionnaire starts with the section titles in the color I have set.
Every time a section is completed (even without validation) the color of the corresponding menu item returns to the default value

Perfect.

请问每个组的索引是怎么实现的？非常感谢！^_^