Hi, my Touch ID is broken (not sure why). I got it as a gift, it's a iPhone 5s and the back Is blue where it should be gold. So clearly the back has been changed, so will that void my warranty?
If anyone other than Apple opened the phone (which would include changing the back cover) then the warranty is LIKELY void. You may be able to make the claim that the modification did not cause the issue, but I wouldn't count on it.