<form>
<input type='radio' name='sexo' onClick='HabilitarColorCabello()'>Mujer
<input type='radio' name='sexo' onClick='HabilitarColorCabello()'>Hombre
<br>Color de Cablello <input type='text' id='txtColorCabello' name='txtColorCabello'>
</form>
<script>
function HabilitarColorCabello()
{
var f=document.getElementsByName('sexo');
document.getElementById('txtColorCabello').disabled=!f[0].checked;
}
</script>