Prevent code from running? (JavaScript)

<script language="Javascript">
<!--
if (document.layers) window.captureEvents(Event.KEYPRESS);
document.onkeypress = document_onkeypress;
function document_onkeypress(e) {
	if (document.layers) key = e.which;
	else key = event.keyCode;
	//check for valid values an length...
	//if everything is correct, change the color
}
//-->
</script>

<!--
function changeProp(objName,theProp,theValue) { //v3.0
  if (theValue.length > 5) {
    var obj = MM_findObj(objName);
    if (obj && (theProp.indexOf("style.")==-1 || obj.style)) eval("obj."+theProp+"='"+theValue+"'");
    onError = null
  }
}

<SCRIPT>

	function check_color()
	{
		var frm = document.color;

		// REMOVE BLANK CHARACTERS FROM BOTH ENDS OF INPUT STRING

		frm.cuser.value = remove_end_blanks(frm.cuser.value);

		// CHECK FOR AND VALIDATE HEX INPUT
		var hex = validate_input(frm.cuser.value);

		// IF INPUT VALIDATED THEN CONTINUE WITH WHATEVER
		if (hex.length)
			{
                // PUT YOUR  FUNCTION YOU WANT TO RUN IN HERE

                     alert("YOU HAVE CHOSEN A REAL COLOR");
                     return(false);
			}
		// IF INPUT NOT VALIDATED DISPLAY INPUT ERROR MESSAGE AND CLEAR THE FORM
		     else
			{
			alert("Invalid RGB Color Input Value");
			clear_frm(frm, true);
			}

		// RETURN FOCUS BACK TO INPUT FIELD

		return(false);
	}


	function remove_end_blanks(scolor)
	{
		// REMOVE BLANK CHARACTERS FROM LEFT END OF STRING
		while(scolor.length && scolor.charAt(0) == ' ')
			scolor = scolor.substring(1, scolor.length);

		// REMOVE BLANK CHARACTERS FROM RIGHT END OF STRING
		while(scolor.length && scolor.charAt(scolor.length-1) == ' ')
			scolor = scolor.substring(0, scolor.length-1);

		return(scolor);
	}


	// VALIDATE HEXADECIMAL INPUT ROUTINES

	function validate_input(scolor)
	{
		var hex = "";

		scolor = scolor.toUpperCase();

		if (scolor.substring(0,1) == "#")
			scolor = scolor.substring(1,scolor.length);

		if (scolor.length == 6 && check_hex_chars(scolor) == true)
		   hex = scolor;

		return(hex);
	}


	function check_hex_chars(scolor)
	{
		for (var pos=0; pos<scolor.length; pos++)
			{
			var ch = scolor.charAt(pos);

			if ((ch < '0' || ch > '9') && (ch < 'A' || ch > 'F'))
				return(false);
			}

		return(true);
	}

	function clear_frm(frm, remove_color)
	{

		if (remove_color == true)
		frm.cuser.value = "";
		frm.cuser.focus();
	}


</script>

<FORM name=color>
<input type=text name=cuser value="" size=6 maxlength=6 onBlur="check_color()";>
</form>