If it is the same as a few years ago, the crew (or Captain?) decides whether the Lights have been seen. They can be very faint, and barely gray... and that would "count". If nothing is seen, then it was something like a shorter trip in (I think) a low-cost cabin. But Hurtgrutin does state this policy on their website (or they certainly used to).

And yes, one can arrange for a "wake up call" if they see the lights. But you'd want to be able to get to the deck fast, in case it is very fleeting.

We didn't see any Lights on the NB trip so I don't know what was done for those on one-way, or if it had to be RT for the "deal". But finally, on the SB trip... the call came.

They were not brilliant colors and we were very glad we knew in advance that "gray" was likely.
We did see some faint color, and we also saw the "dancing curtain" shape, so we were very happy.
The astronomer aboard our sailing took photos, and those showed those colors that appear in photos...

GC