Add this to your header:

Hi Ahmed,
thanks for your help. It is a cool feature, but not the one I was looking for.
I would like to have the button displayed as below:
It should be grey (color doesn't matter) as long as the particpant did not answer the question. When he checked a box or entered something it should change to the green version.
It should be some kind of indication for the participant that he met all the requirements to continue.
It would be cool to use the feature for any question type.

Thanks, Sven

Just checking whether a participant has answered or not, is quite different from checking whether they have met all the requirements. In the latter, you'll need to specify the requirements of the answer. However, for the former, you can just use the getSelectedChoices method to change the colour of the next button.