I managed to get my invoices visible in the calendar by the due date, but also I want to make them visible in a different color by a custom field that its editable by the user too, this field is a date field and for example, if the customer didn't pay at the due date, the user can edit the custom field and set a different possible payment date, so the invoice is visible in a different color in the calendar, the objective is to remind the user that the customer is gonna pay him that week, but the color means that is a late unpaid invoice

Calendar code looks like this, I want to add my custom field here so it shows the invoice in a different date too

<?xml version="1.0"?>
<calendar string="Invoices" color="journal_id" date_start="date_due">
                    <field name="partner_id"/>
                    <field name="number"/>
                    <field name="amount_total_signed"/>
                </calendar>

Cheers