I have some doubts about using "command" in the first sentence. You could mean that they "compel" a beautiful landscape, but it sounds rather strange. You would have to explain your exact thoughts more to make that one work. You might instead say "My windows open upon a beautiful landscape." or "My windows face a beautiful landscape." The second sentence works well enough but the word order is different than the usual case (subject-verb-object). It sounds a bit dated. "Which colours does the rainbow consist of." sounds somewhat less stiff. The third sentence is perfect!

The first sentence is good assuming you did mean more than one window. As Francois said, it does sound a bit formal. I would go with everyone's suggestions for the second sentence as well. But, it is also grammatically correct as it is. The third sentence is good too! I think "which" and "what" are pretty interchangeable.

My windows have a commanding view over a beautiful landscape. What colours does a rainbow consist of? What colour are you going to paint your house?

The first sentence is grammatically correct, but somewhat formal/old fashioned. It might be used, but in normal conversation you would say 'There's a beautiful view from this window." I agree with Francois's suggestion for the second sentence, "Which colours does this rainbow consist of?", and that your last sentence is good.