In iOS 7 you simply can change the slider color of a MPVolumeView
like this:
CGRect rect1 = CGRectMake(17, 45, 220, 0);
MPVolumeView *volumeView = [[MPVolumeView alloc] initWithFrame:rect1];
volumeView.tintColor = [UIColor colorWithRed:0.0f/255.0f green:255.0f/255.0f blue:127.0f/255.0f alpha:1.0f];
Hope this helps.