Why don't you take advantage of the mishap & paint the offending wall a deeply contrasting colour to the other walls? Depending on the existing colour scheme, furnishings etc, having one wall a different colour to the others can be extremely effective in terms of decor. It can be extremely difficult to match an existing shade exactly, even using the same paint. Factors such as the length of time since the original paint job, whether or not there's an open fire in the room, whether you or your family are smokers etc can all affect the degree to which a new paint job will match.