Thanks for the post. This is an interesting issue and not something I’ve been asked before. Unfortunately, there’s no way to color, or otherwise highlight, wireframes in the Navigator panel on the left hand side when in ‘Thumbnail’ mode.
I do have a couple of ideas that might help you better navigate that list:
Use our hierarchy feature to manage the list. Child wireframes can be indented and you can also collapse and expand sections, hiding or showing child wireframes to make the list easier to navigate:
You can read about the hierarchy feature here: Using Mockup Hierarchy
Another Idea I had was to just draw a large colored rectangle over the wireframe you wish to highlight and color that - it’s a single control so it’s easily added and removed and, because the thumbnails reflect the actual wireframes, the color will appear in the Navigator like this:
Sorry to not have a better answer @Ines_Guerbaa but I hope one of these workarounds might work for you?
All the best and happy wireframing!