试试这个：

UITextField *searchField = [self.searchBar valueForKey:@"searchField"];

// To change background color
searchField.backgroundColor = [UIColor blueColor];

// To change text color
searchField.textColor = [UIColor redColor];

// To change placeholder text color
searchField.attributedPlaceholder = [[NSAttributedString alloc] initWithString:@"Some Text"];
UILabel *placeholderLabel = [searchField valueForKey:@"placeholderLabel"];
placeholderLabel.textColor = [UIColor grayColor];