Aileni indulges his Monochrome Mania at this dedicated site, and lets the rest of us play along.
I actually removed color from only three of these, and if I hadn't told you that you would probably not have noticed the tiny bit of color in the one I left. Do you even seen it?
The primary color removed? the blue of the sky. And here I have been, moaning over our gray skies lately!
Removing the color gives an opportunity to look at other aspects of photography--such as composition. The same view above and below--which do you prefer?