Write two same navigation controllers and AuthorViewController

Write two same navigation controllers and AuthorViewController
This is an overcomplicated solution. Try this one:

    NSScanner *const scanner = [NSScanner scannerWithString:hexString];
    if ((hexString.length == 7) && ([[hexString substringToIndex:1] isEqualToString:@"#"])) {
        [scanner setScanLocation:1];
    }

    unsigned hexColor = 0;
    NSAssert(![scanner scanHexInt:&hexColor],
             @"The specified hex string is invalid.");

Once you have a hexColor, it's not that difficult to create a color using the bitwise operations.