There are many possible reasons Grace could be called “Shade”, though none of them stand out. Could she be a ghost? Probably not. Is she wearing cool shades? Nope. Does she spend a lot of time in the shade at the beach? Possible, but it’s night, so it’s difficult to tell. Also, Tedd’s house is not a beach.
In retrospect (as pretty much all my thoughts in these classic commentaries are), it’s a bit odd that Tedd’s dad would be that familiar with Sarah. Yes, Sarah is an “arteest”, and would fancy herself decent at fashion, but how does he know that? Has Sarah spent a lot of time at Tedd’s house dropping fashion knowledge on people?
Then again, I’m probably overthinking it. Sarah’s a girl Tedd knows who looks like she knows how to dress herself. That’s really all the credentials Tedd’s dad needs.
Commentary added August 13