Gist that seeks to emulate a color mask.

Mask UI

//
//  TUIMaskViewController.m
//  TestUIs
//
//  Created by Murillo Nicacio de Maraes on 3/13/14.
//  Copyright (c) 2014 m-one. All rights reserved.
//

#import "TUIMaskViewController.h"

@interface TUIMaskViewController () <UIScrollViewDelegate>

#pragma mark - UIView
@property (nonatomic) UIView *bottomView;
@property (nonatomic) UIView *topView;
@property (nonatomic) UIScrollView *maskView;

#pragma mark - View Properties
@property (nonatomic) CGFloat topCenterInitialX;

@end

@implementation TUIMaskViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    [self setUpView];
}

-(void)setUpView
{
    UIView *clipperView = [[UIView alloc] initWithFrame:self.view.frame];
    clipperView.clipsToBounds = YES;
    
    [self.view addSubview:self.bottomView];
    [clipperView addSubview:self.topView];
    [self.maskView addSubview:clipperView];
    [self.view addSubview:self.maskView];
}

#pragma mark - UIScroll Delegate
-(void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    CGFloat xDelta = scrollView.contentOffset.x;
    
    self.topView.center = CGPointMake(self.topCenterInitialX + xDelta, self.topView.center.y);
}


#pragma mark - Accessors
-(UIView *)bottomView
{
    if (!_bottomView) {
        _bottomView = [self createTemplateView];
        _bottomView.backgroundColor = [UIColor blackColor];
        ((UIView *)[_bottomView.subviews firstObject]).backgroundColor = [UIColor whiteColor];
    }
    
    return _bottomView;
}

-(UIView *)topView
{
    if (!_topView) {
        _topView = [self createTemplateView];
        _topView.backgroundColor = [UIColor whiteColor];
        ((UIView *)[_topView.subviews firstObject]).backgroundColor = [UIColor blackColor];
        
        self.topCenterInitialX = _topView.center.x;
    }
    
    return _topView;
}

-(UIScrollView *)maskView
{
    if (!_maskView) {
        _maskView = [[UIScrollView alloc] initWithFrame:CGRectMake(0.0, 0.0, CGRectGetWidth(self.view.frame), CGRectGetHeight(self.view.frame))];
        _maskView.contentSize = CGSizeMake(2 * CGRectGetWidth(self.view.frame), CGRectGetHeight(self.view.frame));
        _maskView.pagingEnabled = YES;
        _maskView.bounces = NO;
        _maskView.showsHorizontalScrollIndicator = NO;
        _maskView.delegate = self;
    }
    
    return _maskView;
}

#pragma mark - View Creation
-(UIView *)createTemplateView
{
    UIView *containerView = [[UIView alloc] initWithFrame:self.view.frame];
    
    UIView *contentView = [UIView new];
    contentView.bounds = CGRectMake(0.0, 0.0, 280., 40.);
    contentView.center = containerView.center;
    
    [containerView addSubview:contentView];
    
    return containerView;
}

@end