[Dilettante_Pro]

без аякса, чекбоксов и радио. Сколько угодно вариантов цвета

<div id="blok" style="width:50px;height:50px;border:1px solid black"></div>
<form>
   <input type="text" id="currentcolor" name="currentcolor" value="white" style = "display:none;">
</form>
<script>
var color = ["white","green","yellow","red"], i = 0;
blok.onclick = function() {
   i = i < (color.length - 1)?i+1:0;
   this.style.backgroundColor = color[i];
   currentcolor.value = color[i];
}
</script>