What actions might change the value?
The current user edited the field?
If another user edited the field?
New data was imported into the file using the update option?
It's a calculation field and a field it references was modified?
If the first and/or second options are the only ways you need to flag this change(direct edit on the field), then for how long would it be "flagged"?
Scripting with a script trigger and a conditional format can be used in concert to produce a color change if the value of a field is modified by editing it on the same layout, but the details can vary depending on what you need.