The umbra part of the sunspot is cooler, but each sunspot also has a penumbra region which causes the sunspot to be brighter. This is due to the magnetic field

The umbra part of the sunspot is cooler, but each sunspot also has a penumbra region which causes the sunspot to be brighter. This is due to the magnetic field generated that causes the sunspot to overcompensate. This is the reason some theorize the Little Ice Age may have been caused by the Maunder Minimum.