if checkbox is checked, do this

I would use .change() and this.checked:

$('#checkbox').change(function(){
    var c = this.checked ? '#f00' : '#09f';
    $('p').css('color', c);
});

—

On using this.checked
Andy E has done a great write-up on how we tend to overuse jQuery: Utilizing the awesome power of jQuery to access properties of an element. The article specifically treats the use of .attr("id") but in the case that #checkbox is an <input type="checkbox" /> element the issue is the same for $(...).attr('checked') (or even $(...).is(':checked')) vs. this.checked.