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.
Try this.
$('#checkbox').click(function(){ if (this.checked) { $('p').css('color', '#0099ff') }}) Sometimes we overkill jquery. Many things can be achieved using jquery with plain javascript.
It may happen that "this.checked" is always "on". Therefore, I recommend:
$('#checkbox').change(function() { if ($(this).is(':checked')) { console.log('Checked'); } else { console.log('Unchecked'); }});