Linux

Well, OS has nothing to do with rectangle coloring :)

I suppose that is true. I think most people view "the OS" as synonymous with the kernel and what associated applications enable you to do (obligatory GNU/Linux copypasta). That's usually how I think of it, though I know it's not accurate.

It's a bit like including the sinks and shower when you talk about "the plumbing" in your house. Depends on your experience and what level of abstraction you're thinking in. I see by your comments that you're using a much more rigorous definition of "OS." :)