My answer is a good starting point, some of the other answers provide flexibility for different situations. If you care about IE8, look at the old version of my answer.
<h1><span class="grey">1</span>A grey circle with number inside</h1>
<h1><span class="red">2</span>A red circle with number inside</h1>
<h1><span class="blue">3</span>A blue circle with number inside</h1>
<h1><span class="green">4</span>A green circle with number inside</h1>
<h1><span class="pink">5</span>A pink circle with number inside</h1>