I'm mystified at your description of a cardigan as 'underwear'. God knows, nobody accuses Norwegians of being overly bright but you'd think they'd at least understand how to wear clothes properly. You're all so in-bred that you can't take the advice of well-meaning 'outsiders'. You probably wear your clogs on your head too.
Go find a picture of Charles I's undershirt -- it is nasty so I don't want it in the bakeshop. I do not expect you to know the first rule of historical fashion by which underwear becomes outerwear but that is one of the oldest surviving examples, right down to the pattern which, in Norwegian, is known as "
kors og kringle". Norwegian peasants began to wear it under their overalls, peeking out at the top and embellished with embroidery and whatever tin they could manage as buttons, and that is the ancestor of what you have thoughtlessly named a "cardigan", as though you folks invented it. You did not; you just forgot.
The two- (or more) color knitting was for warmth, making a double thickness, and allowed for the incorporation of designs that very likely have their roots in pagan superstition across the breast (Viking-era tablet weaving has the same pattern) and also personalization -- many early examples from Scandinavia have the dates and owners' names worked in.