Things to look for while suit shopping?

As a tailor, if there's one thing to make sure fits you well, it's the shoulders. Minor alterations elsewhere on the jacket are generally easier (read: cheaper for you) should they need to be done in the future.

I'm not familiar with suit supply, but what I would say is the most important thing is to go in with an open mind, look at different options, and if you aren't one of the few people who can dress off the rack, make sure they offer tailoring to make small adjustments.

I highly recommend looking at lighter shades of you already have a darker grey. That and look at shirts and ties of different colors to mix and match. It might seem outside your comfort zone, but a light whitish grey suit with a purple shirt or tie, or other splashes of color really look nice.

I wear suits a few times a week. Sometimes you just want to blend in and wear a dark suit if that is what others are wearing. But you lose a lot of personality always wearing dark colors to work or formal events. On days with big important meetings I dress conservatively, and other days add lots of colors.

Also vests are a great way to look formal while ditching the jacket that can be oppressive in the summer. And a great with jacket will set you apartb in formal settings.

Before you go, wear on the shirt and shoes you plan to wear with the suit.

Look carefully at how the suit fits. You can always get it tailored so the pants fall just right on your shoes and the cuff of your shirt shows, but you want to avoid odd creases on the back and front when the suit is buttoned up. Those are harder to tailor out.

Also, think carefully about if you want to buy a well tailored suit for now, or if you want to buy a suit that can evolve as you do. When I have a perfectly tailored suit, then put on weight, it is no longer perfectly tailored. Eventually the pants no longer fit, and now that jacket is useless. It's not such an issue at 20 hopefully, but once you hit 40, things might change. Brooks Brothers lets you mix and match pants to suit tops. It means you can get the perfect pants to go with the perfect suit top with minimal tailoring. You can also get the next size up pants just in case.

A few scattered thoughts.

On suit color - You mentioned you already have a dark grey suit, unless you are going (or expect to go to) a formal black tie event or a funeral anything you'd need a black suit for a dark grey/charcoal one would cover. IF you want another suit a lighter grey might give you more versatility, though black is a classic for a reason so if you'd really like to have one in your closet it absoltuely doesn't hurt.

Patterns - If you only have a few suits, I'd PROBABLY suggest you stick to something classic as opposed to getting a cut or fabric that's too clubby/fashion forward. Getting any overly funky fabrics or an exotic options will limit how many places and situations you can wear your suit as well as the lifespan the suit will look good on you, but only you know your social circle and the palces you might wear this.

Shirts - If you're into fashion you probably have an eye to what looks good in general and for you in particular. You absoltuely want at least one white shirt, and in general you want to do the opposite of what you do with your suit fabric here; if you have a plainer fabric on the suit you can do something a bit more busy like a pinstrip or herringbone pattern with the shirt, if your suit itself is pinstripped or herringbone go with a simple plain weave and so on. I personally really like Charles Tyrwhitt for the fit and value, but you can go with most any button down that works for you. Pocket is optional (though I most always get one), and buttons on the collar are more informal than one without; everything else feel free to go with whatever works for you

Belt/shoes - You can just google suit color combinations to see a ton of different graphics people have made for what does and doesn't work, but don't cheap out on these. The belt doesn't really matter too much as long as it's leather (or leather equivalent quality if that's your moral stance), but good shoes matter a lot in how you look overall and are worth budgeting in too, especially since a good pair lasts basically forever and you can use them for other outfits and situations. If you go with a black suit you basically have to go with black shoes/belt (unless it's a high fashion sort of crowd, but if you were comfortable at that level you wouldn't be asking), but for your dark grey suit and if you go with a lighter color brown works as well.

Fit - This goes for most everything you'd wear, but as suits are an even more extreme example of the fact even an amazing quality suit will look horrible if it doesn't fit right. Most places have an in house tailor, but this is important enough it might well be worth calling a couple of the best reviewed stand alone places in your area to see what they'd charge to tailor a suit, it's better to get this perfect than to get it done cheapily or more conveniently by someone who's right there but not great.

Tie - The tie (and a pocket square, if you went that route but that can look too try hard/fussy depending on the crowd/situation) is basically the only color you're going to have. You CAN do anything you want, if you're confident in your look and can in general pick "this looks good together" type color/style combinations, but for the most classic look the same way your shirt should mirror your suit fabric (plain on pattern, patterned on plain), you want your tie to mirror your shirt. As a general rule of thumb I actually prefer second hand here, I have a decent selection of ties and most every one I found by finding a list of good tie brands, then searching them up on ebay and finding ones I liked that were selling for cheap. If you don't have that kind of time or want something new tiebar.com is actually great, their stuff is surprisingly nice for dirt cheap and they have most every style and color you could possibly need. This is basically your only pop of color, so a few nice options with a couple different shirts can make it seem like you havfe a whole closet of suits.