Clouds are a group of consdensed tiny water droplets or ice crystals.
Clouds usually appear white because they reflect most of the sunlight. However, as the clouds get denser i.e contain more water droplets, they absorb more of the sunlight and reflecting back less, causing them to appear grey.
Photos taken with my Samsung Galaxy J4.
Here's my previous post in white and grey clouds.
https://steemit.com/science/@a-condor/why-clouds-appear-grey-or-white