<p>
<input type="radio" name="cor" value="1" id="c1"><label for="c1">Vermelha</label><br>
<input type="radio" name="cor" value="2" id="c2" checked><label for="c2">Verde</label><br>
<input type="radio" name="cor" value="3" id="c3"><label for="c3">Azul</label>
</p>
input[type="radio"]:checked + label { color: magenta; }