The question I get asked most about grey bags is whether they're just a sad compromise between black and beige, and honestly, no. A good grey bag does something neither of those does, it softens an outfit instead of anchoring it, and once you've used one for a few months you notice black feels heavy by comparison.
The shades that work hardest are the ones that lean slightly warm, a dove grey or a greige, rather than anything with a cold blue undertone, which can end up looking like you've accidentally brought a laptop bag to a wedding. A soft charcoal is the other safe bet, dark enough to do what black does but with none of the starkness. I had a pale silvery grey bag years ago that looked gorgeous in the shop lighting and then turned faintly lilac under anything else, so I'd always check it in daylight before you commit, not just under shop spotlights designed to flatter everything.
What actually separates a decent grey bag from a disappointing one is rarely the colour at all, it's what's holding the colour up. Cheap grey often comes on a flimsy faux leather that creases within a fortnight and never drops back out, so the bag looks tired by week three. Look at the stitching along the base and handles, that's where mine have always failed first, and check the hardware isn't a bright shouty gold that fights the softness of the colour, a brushed silver or gunmetal sits much better. A proper structured base matters too, grey shows every dent and slump more than black does, there's nowhere to hide.
Where it struggles is against anything already muted and pale, a stone coat or an oatmeal jumper can make a grey bag disappear into the background rather than finish the look. I made that mistake with a whole outfit once, cream coat, beige trousers, grey bag, and ended up looking like I was dressed in fog. It wants a bit of contrast, navy, black, a rich green, even denim, to actually register as a bag rather than a shadow. Pair it with jeans and a white shirt and it looks instantly more put together than the same outfit with a black bag, which is the detail that sold me on grey in the first place.
Mine has gone from school run to a dinner out to work meetings without ever feeling wrong for any of them, which is rarer than it sounds in a bag that colour. It's not flashy, it just quietly fits into whatever you're already wearing most days. If you're someone who reaches for the same three or four outfits on rotation, it's worth having one in the mix.