Your method called by the notification should have the signature:
- (void)mySubViewNotification:(NSNotification *)note
(note the lower case first letter - standards - class names start with capitals, methods with lower case, update your code accordingly, and the parameter being a notification)
Then you can add, in the method:
UILabel *label = note.object;
label.textColor = [UIColor whiteColor];
because the notification is passed to you as a parameter and you need to get the enclosed information (the label in this case) from it.