Dark clouds:

* Contain larger water droplets or ice crystals, which absorb more sunlight.

* Appear dark and ominous, often associated with storms or precipitation.

* Are commonly found at low altitudes and are associated with rain or snow.

White clouds:

* Contain smaller water droplets or ice crystals, which scatter and reflect more sunlight.

* Appear fluffy, bright, and cotton-like.

* Are commonly found at higher altitudes and are typically associated with fair weather.

In summary, the difference in the size of the water droplets or ice crystals within clouds determines their appearance, with larger particles leading to darker clouds, while smaller particles create brighter, whiter clouds.