Hey Mason - wow. Great questions. The good news is that this is less complicated than you think – and that a pair of blue jeans is already a great starting point. (In fact, more often than not, I also go for jeans with a sport coat...so you're not as far off as you thought.) Beyond that, here's how I think about it.
Expand your definition of 'jeans.' You already know that a pair of jeans is a safe bet. But if you're not counting white jeans there, you should be – jeans that are blue, white, or, most of the time, natural wheat (unless your jacket is wheat; more on this later) are going to look great with any sport coat you throw at them, solid or patterned. I know white after Labor Day is controversial, but all three of those work year-round in my book as long as they've got some heft to them. I love a tweedy autumn sport coat with white jeans and suede boots in, say, November.
Okay, but jeans (or five-pockets; for us, they're the same) may not be appropriate for every occasion. When trying to pick trousers for your sport coat, it's a good idea to stay neutral on the bottom. I tend to stick to earth tones – tans, browns, stones, khaki, even a little green. But the most versatile of all is grey. Grey trousers may be even more of a safety play than jeans. If you think about shades on a spectrum of 1-10, ranging from pale pearl all the way to charcoal briquette... any shade of grey from about 2.5-9 is going to go with pretty much any sport coat. If I'm not wearing a pair of jeans or 5-pockets with my sport jacket, odds are pretty good that I'm in mid-grey or charcoal high-twist. So grey's almost always going to work, no matter what's on top.
After grey, I'd look for a secondary or tertiary color in the pattern to guide you. You just want some connectivity. Like this one's got some tan running through it – so tan trousers, or maybe a char brown, would work well. And this houndstooth would be great with either Air Force blue or medium blue, or even a very light khaki to pick up that off-white. This also works in reverse, by the way, if you've got a pair of patterned trousers – for example, these would look great with a navy or mid-blue jacket. So can you mix patterns with solids? Heck yeah - especially if that solid is grey. (And speaking of tried and true combinations - grey trousers with a navy jacket. Impossible to mess up.)
Now what are the things to avoid? If your jacket is solid, with no extra colors in the mix, you wanna be careful about doubling up on a single color. This means no green pants with a green jacket, no khakis with a khaki jacket, no brown pants with a brown jacket. And if you happen to own a plain white jacket (hmmmm) you wouldn't wanna wear that with white jeans. You want to go for contrast instead. Green with khaki - khaki with brown – brown with blue – blue with grey - and so on. I mentioned how useful your grey trousers are, and this is one reason we don't make a lot of solid grey sport coats. They cancel each other out. If it's a super textured Shetland or Harris Tweed, maybe, but generally we don't go there. So be careful with the colors getting too close. The magic exception here is blue jeans and a dark blue blazer. I don't know why that works so well, but it just does. Navy trousers – sport or dress – are a no-go with navy on top, but jeans? Always.
Second, don't be tempted to take the suits you already own and wear them as separates. Generally, the only suits you wanna split up are the ones with some texture to them... the fabrics with "touch." Seersucker, linen, canvas, corduroy, heavier wools, something that's brushed like moleskin or flannel. If it's super textured or slubby or beefy, it can usually stand on its own. And that's the key. But I'm guessing that most of the suits in most guys' closets fall into the "don't mix" category. If it's a plainweave fabric, smoother and lighter in feel, you want to keep the pieces together. This glen plaid plainweave is a perfect example. As a suit? Chic as all get-out. But just the jacket on its own doesn't look right – too flat. It needs some heft to it. But once again, plain navy acts as the exception here. You can split up nearly any navy suit you've got, whether it's textured seersucker or a high-twist plainweave. (That's why a navy suit is such a wardrobe MVP.)
flax/grey/tan glen plaid can go with brown, olive, or oatmeal trousers | blue/sage green herringbone can go with charcoal, blue, or light grey trousers
brick/blue houndstooth can go with air force blue, blue, or stone trousers | pine/navy/sky windowpane can go with char blue, mid-grey, or dusty blue trousers
In the end, if you're trying to add some more pattern to your wardrobe, and you don't wanna wear a full suit... don't buy a full suit. I know it may seem more economical (hey – 3 for the price of 1!) but remember that sport coats are designed for this. Just look at this page. Every single one of those can go with a pair of jeans, it can go with grey trousers, and it can usually go with a few more neutrals. We go through waaaaaaayyyy too many fabrics every season to produce a jacket that doesn't give you some range.
The best way to get a feel for the mixing and matching is just by doing it over and over and over again, and you may need a little help in the beginning. Reach out to a friend, or your go-to clothier (hopefully it's us) for some steering, or a second opinion. My friends text me photos all the time – "hey, does this work?" It can be hard, even for me... and I've been putting outfits together since I was twelve years old. I actually have this lightweight flannel Prince of Wales jacket that I wear fairly often, and almost every time, Ann's like "...are there pants that are supposed to go with that?" She's not a menswear expert, but it just looks 'off' to her. I'm not sure she could even tell you why. That said, I've also had some guys tell me "man, that's a great sport coat." Which also goes to show you that some of this can be subjective.
Okay, Mason – start by giving some grey trousers a try, and see how that feels. 99% of the time, it's going to work... and if you need a little reassurance, we're only a phone call or an email away.
