AngularJS Example:

HTML

<link rel="stylesheet" href="http://twitter.github.com/bootstrap/assets/css/bootstrap.css">
<form method="post" action="sample.php" onSubmit="return validator(this)" name="register">
<select name="bcolor" size="1">
<option value="" selected="selected">Pick a Color</option>
<option value="rd">Red</option>
<option value="yl">Yellow</option>
<option value="gr">Green</option>
</select>

<select name="bcolor1" size="1">
<option value="" selected="selected">Pick a Color</option>
<option value="rd">Red</option>
<option value="yl">Yellow</option>
<option value="gr">Green</option>
</select>
<input type="submit">
</form>

JavaScript

function validator(theSelect)
{
    var bcolor = document.forms["register"]["bcolor"];
    if (bcolor.value == "") {
        alert("Please Select Your color1");
        bcolor.focus();
        return false;      
    }
    var bcolor1 = document.forms["register"]["bcolor1"]["bcolor"];
     if (bcolor1.value == bcolor.value) {
         alert("Please Select Your color2");
         bcolor1.focus();
         bcolor.focus();
         return false;       
     }
     if (theSelect[theSelect.selectedIndex].defaultSelected)
        return true;
}