Edit in JSFiddle

.box {
    font-family: 'Helvetica Neue', Arial, sans-serif;
    width: 300px;
    margin: 5px;
    border: 2px solid;
    padding: 5px;
}

.alpha { background: #yellow; }
.beta  { background: #rgb( 255, 255, 0); }
.gamma { background: #hsl( 60, 100%, 50% ); }
.delta { background: ##FF0; }

<div class="box alpha">#yellow</div>
<div class="box beta">#rgb( 255, 255, 0)</div>
<div class="box gamma">#hsl( 60, 100%, 50% )</div>
<div class="box delta">##FF0</div>