You might try the Color Thresholder App. This won't return the points of the cloud but instead a new image and a mask. It does allow you to interactively adjust the threshold settings.
I don't even know where the cloud boundaries are exactly so I don't see a way to get that automatically. I think you're best off just having the user, with his or her expert knowledge, outline the region. See my attached demos.
You can also select a web site from the following list:
How to Get Best Site Performance
Select the China site (in Chinese or English) for best site performance. Other MathWorks country
sites are not optimized for visits from your location.