The Black Watch tartan, dark green and navy crossed with black, takes its name from the Highland companies raised in 1725 to keep watch over the Highlands. It is still one of the best-known tartans there is, and it is the reason forest green and navy look so natural together. You have seen the pairing on kilts, blankets and school uniforms your whole life.

Without the plaid, the two colors are just as easy. Both are dark and cool, so the effect is quiet and a little preppy, never loud. I treat navy like black and forest green like the color, but in low light people will barely tell which is which, and that is part of the appeal.

A quick test: put a navy sweater next to a green one in daylight. If both look rich rather than murky, wear them together.

Bottega Veneta's small Dustbag pouch is green Intrecciato leather, and Gianvito Rossi's Lisa ankle boots are emerald suede. For navy, Manolo Blahnik's Maysale mules are navy suede with a buckle, and Khaite's Olivia mini bag is midnight blue suede with a top handle. Navy accessories work like black ones, so they go with almost everything.

The Frankie Shop's ribbed half-zip is a green wool and silk blend, and The Row's Eglitta pants are high-waisted navy corduroy with a wide leg. Jimmy Choo's Margot ballet flats are forest green suede, and Kallmeyer's Driving tote is navy canvas trimmed in leather. It is the kind of outfit you can wear to a meeting and then straight to dinner.

Rùadh's Oscar sweater is navy merino with embroidery, and it sits well over Fforme's Jovie midi skirt in forest green crepe satin, which has a soft shine. Jimmy Choo's Cycas ankle boots are navy suede, and Dragon Diffusion's Santa Croce tote is woven green leather.

How to wear it: keep the sweater untucked if it is thick, and tuck just the front if it is thin. The skirt should still show at the hip.

Carolina Herrera's ruched wool-blend midi dress is a deep green, and Max Mara's Baobab blazer is navy pinstripe seersucker that goes right over it. Christian Louboutin's Turela ankle boots are navy suede, and Jia Jia's Arizona necklace hangs green quartz in gold.

What I would skip: a green bag as well. With the dress and the necklace, a navy or black bag keeps it from looking like a uniform.

Loewe's twill blazer is a dark green wool blend, and it goes over a column of navy: Róhe's cotton-poplin shirt and La Ligne's striped silk-satin wide-leg pants. The Row's Adam penny loafers in dark green glossed leather bring the blazer color down to the floor. It is the most polished outfit here and the one I would wear on a big day.

If you already wear a lot of navy, forest green is the easiest new color you could add. It goes with the same denim, camel and gray, and it reads as dark from across a room. Start with one green knit or a pair of green flats and wear them with the navy you own. Once that feels normal, try the green blazer. If you want to keep browsing, I also put together Navy and Cream Outfits to Try This Fall and Rust and Navy Outfits to Try This Fall.