1.如果只是设置选中状态的字体颜色，使用 tintColor 就可以达到效果
self.tabBar.tintColor = [UIColor redColor];
2.但如果要将未选中状态和选中状态下的颜色都改变，可以使用 setTitleTextAttributes:<#(nullable NSDictionary<NSString *,id> *)#> forState:<#(UIControlState)#> 达到效果
[[UITabBarItem appearance] setTitleTextAttributes:@{NSForegroundColorAttributeName:[UIColor redColor]} forState:UIControlStateNormal];
[[UITabBarItem appearance] setTitleTextAttributes:@{NSForegroundColorAttributeName:[UIColor blueColor]} forState:UIControlStateSelected];
