I really can't think of anything... When I get home, I'll swatch Summer Neutral with any pigments I have that are even remotely close, but I seriously can't think of any...
::: ETA ::: Okay, as soon as I got home (before I forgot), I swapped Summer Neutral, and then looked through my pigments to see what might be a dupe. Needless to say, I can't think of any and nothing came close. I'm going to check on something, and I'll get back to you.