How can I change the small map in the sidebar of the listing details page, from black & white to colour?
You can make the sidebar map color by removing the grayscale styling from the map’s settings. It’s usually controlled by a custom JSON style in the Google Maps setup. Check your theme’s map template (like single-listing-map) and remove or adjust the styling. More on map styles here: Style Reference for Maps JavaScript API | Google for Developers
Sometime I don’t even understand how Google Maps work, step aside I don’t think a regular user would be able to “modify” the code. 
Sorry but this advice is useless as if I tell you my car doesn’t start, it’s like sending me the details of the “engine” how it works.
Thanks for the input, it’s a tough one but possible. I searched file manager & can’t find the file to edit, it really should have been included in theme options.
Contact author and ask for help, if it’s not provided by the author, you will need customization.
In that case, if you need someone to help you ( paid job ) I’m available, you can contact anytime.