Edit in JSFiddle

body {
    background-image: url(http://sachips.byeto.jp/demo/images/bk01.png);
}

.touka01 {
    padding: 10px;
    background-color: #ff0000;
    filter:alpha(opacity=15);
    -moz-opacity:0.15;
    -khtml-opacity:0.15;
    opacity:0.15;
}

.touka02 {
    padding: 10px;
    background-color: rgba(255,0,0,0.15);
}

<p class="touka01">背景も文字も透過</p>

<p class="touka02">背景のみ透明化（文字は透過しない）</p>