It kind of depends whats causing the discolouration. If its yellowing perhaps due to surface oils, you can normally do something and get less of a difference. Sometimes if its due to one part of the plastic fading due to exposure to the elements, then by covering it up and only exposing the other part you, again, can get a modicum of similarity. But if its simply the 2 different plastic batches ageing and therefore changing colour (or perhaps having been different colours at the point of manufacture) at their own speed and way unfortunately you cant do anything.