I was one of the black phone / white charge plate / yellow charge cover customers. After online chat with O2 getting nowhere (an hour of my life wasted!), I emailed O2 setting out exactly why I felt it was their responsibility to correct what was very clearly an error. Irrespective of whether it was Nokia's fault or O2's, the supply of the free accessory kit was part of the deal which O2 had offered me to get me to pre-order. In my opinion, that means the accessories are part of a contract between O2 and me, and they are therefore obliged to deliver on the deal. Supplying inferior goods (and again in my opinion, supplying a yellow accessory in a box that says black accessories, is an inferior product!!) means they have short-changed me and are responsible for correcting that.
I got an explanation as to why O2 could not supply a replacement cover, but the good news is that they agreed to reimburse me for the cost of buying a black one as a goodwill gesture, which they have now done. Result! 
