Yes,
I am using a Full With page with inside container of 1200px.
So, how can I put a custom full width background color for a H2 heading using this structure https://ibb.co/PWVzwfV?
Since you have and inside container of 1200px, you can place the H2 within a Container Block itself outside of the Container Block you have, and the set the Container and the Inner Container to Full Width.
Otherwise, if you follow the current structure you have, you can just use the WP width setting, and set it to full width as such: https://share.getcloudapp.com/ApuXzXK5