I think you can find the answer to your question here (it is from the NOAA website, so I consider it reliable):
Sometimes, under direct sunlight, clouds will appear grey or dark grey against a blue sky or larger backdrop of white clouds. There are usually two reasons for this effect:
- The clouds may be semi-transparent which allows the background blue sky to be seen through the cloud. Thereby giving it a darker
- A more common reason is the contrast between the background (blue sky or additional clouds) and foreground cloud overwhelms our vision.
In essence, our eyes are tricked with our perception of foreground
clouds appearing dark relative to the overwhelming brightness of the
So, point 1 is basically saying that from those thinner clouds you have some white light from Mie scattering, but this is not much and it is mixed from blue-ish light from Rayleigh scattering coming from behind (notice that even if in the picture "behind" part of the gray cloud you see a white cloud, there is still air between the two, and that air scatters blue-ish light).
I have to say that I am not really convinced by point 2. I mean, it is clearly not some kind of optical illusion: the two kinds of clouds you show in the picture really have different colors. What is true, though, is that if you were to look at the thinner clouds from another place/angle you would probably perceive them as white.