The resin can absolutely be left in the vat provided the lid is on. If the printer is enclosed and not subject to sunlight then this can be indefinite.  The primary issue with resin sitting idle, especially colored resins, is that it will need to be mixed before printing. This can be done in the vat, but externally stirring the resin ensures that it is mixed thoroughly and prevents the FEP from being damaged. If you leave the resin in after prints, make sure to remove cured resin from the vat before your next one!