Masonry posts in different colours

I have searched for similar topic but found none.
I changed my news start page to masonry look and i like it.

I would like to achive such view that masonry posts blocks would be on white bacground.
The background undeneath would be i.e. gray.
Sth like this:

I tried to set this in settings but cannot do it.

I think there is a custom css for this needed.