Use the hallway opening as your breaking point. Either left or right of it will be the separation (the choice is up to you)

I would tape off an area to paint just as a back drop for the art images, not the whole wall.

OR leave the top of the doorwar gray and start painting the hallway are a couple inches from the top of the door a different color. So it look intentional.

?

Add a panel over one side of the doorway. The other side will look recessed. To make it easy for you, do the panel over the smaller wall. A MDF Fiber Board Sheet would give the wall enough low relief to make it an accent. Darker green on the board, meeting the lighter green over the door. Both colors go to the ceiling.

Don't.

I would colour block the wall leaving a border of the wall colour. No I would not do a line up the wall as your image shows. Here is how it should be done.

IMO a foyer is the introduction to your home so should be in the style of the home so wahy not actually paint that whole wall the color to make it part of the home