An opaque navigation bar with an arbitrary background color and single pixel divider color on iOS 7+. This is more faff than it should be.

NAHMainTable.h

#import <UIKit/UIKit.h>

@interface NAHMainTable : UITableViewController

@end

NAHMainTable.m

#import "NAHMainTable.h"

@interface NAHMainTable ()

@end

@implementation NAHMainTable

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [self hairlineNavigationBarLineWithFillColor:[UIColor whiteColor] strokeColor:[UIColor blueColor]];
}

- (void)hairlineNavigationBarLineWithFillColor:(UIColor *)fill
                                   strokeColor:(UIColor *)stroke {
    const CGRect slice = CGRectMake(0.0f, 0.0f, 1.0f, 128.0f);
    // Assuming that all colors are opaque. Paramaterise this if need be.
    const BOOL opaque = YES;

    // Create a new context to turn into the background image.
    UIGraphicsBeginImageContextWithOptions(slice.size, opaque, [UIScreen mainScreen].scale);
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    
    [fill setFill];
    CGContextAddRect(ctx, slice);
    CGContextFillPath(ctx);
    
    [stroke setStroke];
    CGContextMoveToPoint(ctx, 0.0f, slice.size.height);
    CGContextAddLineToPoint(ctx, slice.size.width, slice.size.height);
    CGContextStrokePath(ctx);
    
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    
    // Set the background image
    [self.navigationController.navigationBar setBackgroundImage:image forBarMetrics:UIBarMetricsDefault];

    // Set an empty image for the shadow image, otherwise we'll get a double line
    [self.navigationController.navigationBar setShadowImage:[[UIImage alloc] init]];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
    // Return the number of sections.
    return 0;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    // Return the number of rows in the section.
    return 0;
}

@end