How can I match the wall color after dent fixing?
Your best shot at that would have been to take a large enough chip from the damaged portion of the wall before it was repaired, take it to a paint store to have it computer color matched. There would have been only a chance it would been an exact match still. 

Your next best bet if it is close enough, is to paint the wall corner to corner, at least it would not be the whole room.