// 默认
NSMutableDictionary *attrs = [NSMutableDictionary dictionary];
attrs[NSFontAttributeName] = [UIFont systemFontOfSize:12];
attrs[NSForegroundColorAttributeName] = [UIColor grayColor];

// 选中
NSMutableDictionary *attrSelected = [NSMutableDictionary dictionary];
attrSelected[NSFontAttributeName] = [UIFont systemFontOfSize:12];
attrSelected[NSForegroundColorAttributeName] = [UIColor darkGrayColor];


UITabBarItem *item = [UITabBarItem appearance];
[item setTitleTextAttributes:attrs forState:UIControlStateNormal];
[item setTitleTextAttributes:attrSelected forState:UIControlStateNormal];