"You have become red in the back of your neck."
Translation:Du er blevet rød i din nakke.
I have translated it directly: "du er blevet rød i bagsiden af din hals" and got incorrect :( But why?
That would be "Du er blevet rød på bagsiden af din hals."
I don't know if your version should be accepted, but "nakke" is certainly the best translation of "the back of the neck".
Nakke is on the same side of you as your back, while hals is on the same side as your face.
If you know Finnish, these are niska and kaula, respectively.