The norm in law is green text and underline for insertions and red text and strikethrough for deletions. I believe GitHub prose diff does green background and underline for insertions and red background and strikethrough for deletions.
I would strongly recommend combining color and text decoration. I have no strong opinion about whether to color text, background, or both, as long as the approach for insertions and deletions is the same.
It might also be worth thinking about dark mode. I just pulled up a GitHub prose diff, and it’s hard to see the styling on the dark mode background grey.