What's the difference between ‘谁是你？’ and ‘你是谁？’?
‘谁是你？’= "who (which one) was you" - For example,  your friend looking at a picture in your high school yearbook, wondering which one in the photo was you at the age of sixteen.

To avoid confusion, it is better to say "哪一个是你" (which one was you) instead of  "谁是你?"

‘你是谁？’ = "who are you?" (if you know English, you understand what it means)

One more Example:

'在这张照片的二十人中, 谁(哪一个)是你?' - Among the twenty people in this photo, who (which one) was you?"

Better to use "哪一个" (which one) than "谁"(who) to make thing more clear.