The light that comes from the sun is white, meaning it’s made up of all the different colours, and we see things as certain colours when those things reflect light of that colour towards our eyes.
The water droplets and ice crystals in clouds are good scatterers (or reflectors) of light of all colours, so we see them as white. Clouds usually look grey when they are in the shadow of other clouds, or in the shadow of themselves. So for instance, clouds look grey when they cover most of the sky because they are blocking a lot of the sunlight. Or large clouds (like cumulonimbus, thunderstorm clouds) look grey at the bottom because they are in their own shadow. If you see a large cloud from far away (or in an aeroplane looking down at clouds) you will see that most of the cloud away from the base will still be white.
Though I will add that snow clouds always look very grey even though they’re quite small & thin, I think this is due to the snowflakes inside them blocking the light a lot more than the same amount of water drops.