Dear Sir.
I am writing this letter to apologize to you for the mistake that we sent the shoes in wrong color. I have acknowledged that my fault has brought you great trouble and unconviniences. I hereby express my deep sorrow for this matter and we are going to do our best to make it up to you. I assure you that this mistake will not happen once again and would appreciate very much if you could give us a chance to show our sincerity on this matter.
Best regards