I wanted to get the grey version because I wasn't a fan of having two different colors, but when I checked Amazon UK after a GAFfer's tip it said the neon one would be in stock a few days earlier, so I went with that. Ended up loving the blue/red Joy-Cons (though my blue has signal issues, so I need to open it and put some foam inside), and I can't imagine owning a gray pair now.
In about a week I'll receive the green/pink combo, so there'll be more options to pick and mix. It'd be nice to own all colors eventually (including grey), but I'm not sure if that's a wise way of spending my money.