So the moral of the story is 1. yes, you are allowed to buy gorgeous clothes, even though you sew and 2. sewing ALWAYS comes in handy!
For the dark grey version I decided to use fold-over elastic - I like the professional looking edge it produces - and decided to document the process with some photos here. I didn't bind the neck edge of all the tees this way: for very fine fabrics I find the elastic too stiff and harsh a contrast. Also, I only have black and white in my stash. Anyway, here is the very straight forward technique on binding a neck edge with FOE.
HOW TO BIND A NECK EDGE WITH FOLD-OVER ELASTIC
3. Edgestitch from the wrong side, stretching the FOE as you go. This is trickier than it sounds - take it slowly and line the elastic up carefully, the end result relies on this bit being accurate.
6. The finished neck edge.