Unfortunately, to get the correct colour mixed up into paint, you need a paint match done on a sample you take into a store. Since that's not an option at the moment, you'll find all of Dulux's shades online to browse through. The likelihood of finding the exact colour without a colour match is not great. It'd be best to paint the whole wall and any adjoining walls. That way, any discrepancy won't be so obvious.
You don't still have the tin from the paint originally used to paint the wall, do you?