function check48(n) {
   if ("" === n) { return true; }
    return n >= 48; }

  function check6(n) {
   if ("" === n) { return true; }
// use the modulus operator '%' to see if there is a remainder
    if (n % 6 != 0) {
     alert('Order by half-dozen increments greater than 48');     	return false;
}

return true;
} 



 // Attach your form's onsubmit handler
var f = document.getElementById('erb'); 

f.onsubmit = function() {
	var el = document.getElementById('BLK'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('BUR'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('DEN'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('HUN'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('JADE'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('KELLY'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('KHAK'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('MEDGR'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('MIDBL'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('MOC'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('NAVY'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('NMEG'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('PURP'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('RED'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('ROY'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('SAGE'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('TEAL'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	el = document.getElementById('WHT'); 
	if (!(check6(el.value) && check48(el.value))){
	return false;
	}
	return true;
};