Here's why:

* Cumulus clouds are made of water droplets. These droplets are small and reflect sunlight very well, giving them their bright white appearance.

* They are often thick and puffy. This thickness allows for more sunlight to be reflected back to our eyes.

* They are typically found at lower altitudes. The lower the altitude, the more direct sunlight the clouds receive, contributing to their brightness.

While cumulus clouds are generally bright white, they can sometimes appear gray or dark if they are thick enough to block out much of the sunlight.