[css] Bootstrap: change background color

You can target that div from your stylesheet in a number of ways.

Simply use

.col-md-6:first-child {
  background-color: blue;
}

Another way is to assign a class to one div and then apply the style to that class.

<div class="col-md-6 blue"></div>

.blue {
  background-color: blue;
}

There are also inline styles.

<div class="col-md-6" style="background-color: blue"></div>

Your example code works fine to me. I'm not sure if I undestand what you intend to do, but if you want a blue background on the second div just remove the bg-primary class from the section and add you custom class to the div.

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.blue {_x000D_
  background-color: blue;_x000D_
}
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<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/>_x000D_
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<section id="about">_x000D_
  <div class="container">_x000D_
    <div class="row">_x000D_
    <!-- Columns are always 50% wide, on mobile and desktop -->_x000D_
      <div class="col-xs-6">_x000D_
        <h2 class="section-heading text-center">Title</h2>_x000D_
        <p class="text-faded text-center">.col-md-6</p>_x000D_
      </div>_x000D_
      <div class="col-xs-6 blue">_x000D_
        <h2 class="section-heading text-center">Title</h2>_x000D_
        <p class="text-faded text-center">.col-md-6</p>_x000D_
      </div>_x000D_
    </div>_x000D_
  </div>_x000D_
</section>
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