The colors in the cells show the status of the changes: green means you have changed the cell but not saved yet, red means the value is invalid, and yellow with a person icon means another user has changed the cell.
The colors explained
Green: You have changed the value, but it is not saved yet. You can undo with Ctrl+Z (Cmd+Z on Mac).
Red: The value is not valid, for example text in a number field or an account number that already exists. Hover over the cell to see the reason. As long as a cell is red, you cannot save. Correct the value or undo.
Yellow with a person icon: Another user has saved a change in the same cell while you had an unsaved change. Below the row, select Get their version or Keep mine.
Muted (grey) text: The field cannot be changed.
Good to know
You can also find the explanation of the colors in the help panel. You open it with the question mark Legend at the bottom right of the table.