If I could only add one colour to a black and ivory wardrobe, it would be red. Not a red dress, not a red coat, just one red thing: a bag, a pair of flats, maybe both if the rest of the outfit is very plain. It is the easiest way I know to make clothes you have worn a hundred times look new.

The key is choosing a red that suits you. Blue-based reds, the kind that lean slightly toward raspberry, look sharp against black and cold light. Warmer, tomato-leaning reds feel softer next to cream and camel. Neither is better, but mixing the two in one outfit can look off, so I try to keep the bag and the shoe in the same family.

The bags start with By Far's Mini 2.0 in semi patent at $315 and Dragon Diffusion's woven Rosanna at $500, and go up to Alaïa's Le Teckel and Métier's Parallel 22 at the top end.

A red shoe is the smaller, slightly braver cousin of the red bag. It draws the eye down, so it works best when your trousers stop right at the ankle or just brush the top of the foot.

Ballet flats are the gentlest way in. St. Agni's satin flat in rosso is $139, and Nili Lotan's leather version has a small bow at the toe. If you like a little height, Khaite's Loop kitten heel in lipstick suede and Manolo Blahnik's Carolyne slingback are both low enough to walk in. Roger Vivier's Trompette and Gucci's Signora sit on sturdy low heels with a bit of hardware, which I love with plain black tailoring.

The one I would think twice about is a high, shiny red stiletto for daytime. It pulls the whole look toward evening, and it is very hard to make it feel casual with jeans and a sweater.

For red to look this good, everything around it has to step back. That means black and ivory, in simple shapes, with nothing printed and very little shine.

Think of a black cashmere crew like Nili Lotan's Boyfriend sweater, an ivory cardigan buttoned all the way up, or a chalk-white top under a black trench. Nili Lotan's Tanner trench is the piece I would build around, since a red bag on the arm of a black coat is about as good as winter dressing gets.

On the bottom half, I like wide, soft trousers in ivory, like Nili Lotan's Flavie silk pant, or a black wool pant that sits on the top of a red flat. St. Agni's Relaxed Bateau jumper in off-white, at $349, is the kind of knit that works with all of it.

None of these pieces needs to be new. If your wardrobe is already mostly neutral, you probably own the base, and the red is the only thing you are adding.