背景の色を指定します。

.sample {
  background-color: #cc9999;
  width: 100px;
  height: 100px;
  border: 1px solid gray;
}

<div class="sample">こんにちわ</div>

• http://www.w3.org/TR/css3-background/#the-background-color
• https://developer.mozilla.org/ja/docs/Web/CSS/background-color
• http://caniuse.com/#search=CSS%202.1%20properties