It depends on how you define "dark storm clouds". Here's a breakdown:

* Nimbus clouds are rain clouds. They are often associated with storms, but not all nimbus clouds are dark.

* Nimbus clouds can be gray or dark. They can be quite dark when they are thick and full of water droplets, especially when they are producing heavy rain.

* Not all dark storm clouds are nimbus clouds. Other cloud types like cumulonimbus (thunderstorm clouds) and cumulus congestus (towering clouds) can be very dark and associated with storms.

So, while nimbus clouds can look like dark storm clouds, they are not the only type of cloud that can be dark and associated with storms.