When opening a modal all fields have some sort of grey color to indicate space for text entry or a calculation result. Canvas cells are completely white on a white background - making them less intuitive to use.
A default background color (or an outline) for canvas cells on a modal would improve the UX.
Using canvas is one of my ways to avoid this grey color 
along with _bold _color etc.
My suggestion has always been for columns which are generated with formulas to be completely white and those that are editable to be, like you suggested, highlighted.
If you dive into the detail view options… its gets really scary and inconsistent how things look.