关于a标签的字体颜色问题

为什么通过类名直接改字体颜色改不了？

<nav class="navbar navbar-default">
    <div class="container container-fluid">
        <!-- Brand and toggle get grouped for better mobile display -->
        <div class="navbar-header">
            <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand" href="#">KingIM</a>
        </div>

        <!-- Collect the nav links, forms, and other content for toggling -->
        <div class="collapse navbar-collapse navbar-changeColor" id="king-navbar">
            <ul class="nav navbar-nav">
                <li class="active"><a href="#">我的好友<span class="sr-only">(current)</span></a></li>
                <li><a href="#">查找好友</a></li>
                <li><a href="#">讨论组管理</a></li>

                <li class="dropdown">
                    <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">更多<span class="caret"></span></a>
                    <ul class="dropdown-menu">
                        <li><a href="#">Action</a></li>
                        <li><a href="#">Another action</a></li>
                        <li><a href="#">Something else here</a></li>
                        <li role="separator" class="divider"></li>
                        <li><a href="#">Separated link</a></li>
                        <li role="separator" class="divider"></li>
                        <li><a href="#">One more separated link</a></li>
                    </ul>
                </li>
            </ul>
            <ul class="nav navbar-nav navbar-right">
                <li><a href="/index/logout">退出登录</a></li>
            </ul>
        </div><!-- /.navbar-collapse -->
    </div><!-- /.container-fluid -->
</nav>

为什么用下面这种方法修改字体颜色不生效？

.navbar-brand{
            color:#fff;
        }

而这种方法会生效？

.navbar-default .navbar-header a {
            color: #fff;
        }

神奇的是，下面这种又不生效：

.navbar-default a {
            color: #fff;
        }

困扰老半天，希望有大神能解答！！！