Hi ! I'm not sure what kind of advice are you looking for . For this window you can use short pair of curtains to avoid covering radiator .Also because window is big I wouldn't advise any dark coloured fabric . Agata

Hi. I'd use a blind rather than a curtain. Go for something in a pale colour if this is the only window in the room. If the room is small I would probably avoid having a black sofa.

Hi. This may be the photo but the room doesn't look that bright so I wouldn't go for something too dark. If you're after leather then maybe a tan colour. If you're after fabric something in a mid tone so grey, taupe, duck egg etc. for the blinds you could look at pale taupe, biscuit, grey, stone, cream etc and then add colour (if you want colour) in your accessories

I wouldn't use short curtains, I think floor length curtains or roman blind would look best. Also I would make the curtain rail longer so that it extends beyond the air vent and hides it from view.
The colours really depend on what colours you like. But go for something light like minnie said, light grey, duck egg blue etc.
I have cream curtains in a cream room and I personally dislike it very much.