Validation Without Alert()

I have the following code somewhat working:

function customerAdd(form){
if (form.email.value == ""){
var email = document.getElementById("email");
email.style.color = "red";
}
if(form.isActive[0].checked == false && form.isActive[1].checked == false){
var isActive = document.getElementById("isActive");
isActive.style.color = "red";
}
if(form.receiveEmails[0].checked == false && form.receiveEmails[1].checked == false){
var receiveEmails = document.getElementById("receiveEmails");
receiveEmails.style.color = "red";
}
if (form.salutation.value == "" || form.salutation.value == "- Please Select -"){
var salutation = document.getElementById("salutation");
salutation.style.color = "red";
}
return false;}

What I meant by somewhat working was that the code actually changes the color of the selected "id" when the fields are empty or have not been selected, but when all the required fields are selected the form doesn't do anything. I knwo it has to do with the return false;

How can I get this to work so that if the user doesn't fill out the required fields they would be highlighted and if everything is filled out the form will submit? I had the code working before with alert boxes, but I wanted to do something different with the way the validation worked rather than having the alert box pop up for each missed field.