Hi,
I have another Idea, if you just want to notify the users that they have atempted a certain question just by looking at the Question Button. You can use the Image button for Submit, the image button can be customized for Up, Hover and Down state. So that would automatically, change the color, when the user hits the button and while visiting thorugh a slide you can get to know by looking at the button.
Alternately, you can keep, Answer All option under Quiz - Settings and it would add a caption to the slide and would make sure that user answers all question (correctly or incorrectly.)
Thanks,
Anjaneai