Highlighting is a good start
But it could conflict with other cell colors if your app supports those (e.g. to differentiate collated calenders). Perhaps instead of using a flat color, use a pattern for unsaved cells? Or play with border thickness/color. Or you could use text formatting if that's not used in the cell design.
At any rate, try to avoid overlapping style types.
Adding an icon will help
Because it will probably fall outside of the style of other cells, which are likely only text-styled. I'd suggest something that relates to the task. E.G pencil (editing), floppy (saving), open/closed lock (solidifying changes).
Use the save button as both input and output.
The button is the way the user will interact a lot and it's pretty big and stands out (literally, in this case). So why not use that to your advantage and make this attention-grabbing piece of screen provide useful feedback?
Perhaps add a short text that explains how many unsaved buttons there are? Have the save button change color or text so it's obvious the context/use changed.
gray [ all cells saved ]
blue [ 2 unsaved cells ]
gray [ 2 cells saved ]
Tie the button to the cells
Use the icon both in the cells and on the button. And use the same color/hue/style on both the cells and the button.
Right now your cells are yellow and the save button is green. Probably just because you quickly made a mockup, but keep it in mind.