This is easy to do thing and imma give you a simple example on how to do it:
1. Go to ACP -> Forums and Posts
2. Choose the forum you would like to color. In the "Options" dropdown, click "Edit Forum".
3. Change the forum title from TITLE to
Sorry, but you need at least 25 posts and 5 hours online time to unlock hidden content.
Replace the color and title with the ones you desire.
4. You're done! Easiest thing ever.
So okay heres a preview
You can simply change it to anything you want, be creative