Medium Gray has less blue in it than Slate.
I've tried to make medium gray a really great go-to basic color for pillows and cushions.
In 2020, I am moving away from #2 or 2mm gauge invisible (what you find at the store). I will continue to stock #2 in shorter zippers (10", 20", 22" for pants & frocks) but have found that #2's loose their strength the longer they are. So think about using either yardage or #4 finished zippers for pillows & #6 invisibles for cushions.