the first key to this is to name your chechboxes the same name, and ending with "[]"
for example:
<inpout type="checkbox" name="color[]" value="red" />
Now in the insert record behavior, when you bind to the checkbox. the code will look like:
<?php echo(isset($_POST['color'])?$_POST['color']:""); ?>
you will need to edit this to use the implode function to convert the array to a comma separated list:
<?php echo(isset($_POST['color'])?implode(", ", $_POST['color']):""); ?>