A strap has only two honest options in front of a dial: repeat a colour already present on it, or step away entirely. The third route — getting close without arriving — is the one almost everybody takes, and the only one that always shows.
A strap blue that sits slightly off the dial blue does not read as contrast: it reads as a failed attempt. The same dial with a black strap works, because black is not trying to resemble anything. It is the first thing I tell anyone who arrives with tone-on-tone in mind.
Black dial
Black is the only dial that accepts nearly everything, which makes it the dullest to dress and the easiest to spoil with a lazy choice. Black on black always works and says nothing. If the watch is a dress piece, fair enough; if it is the only watch you wear, it is a waste.
On black, dark chocolate adds depth without making noise. Cognac changes the character of the watch outright — it pulls it towards daytime, towards casual. Charcoal grey is the most modern choice and the hardest to land: it only works if the indices are pale and cold too, otherwise the watch goes flat.
On black I avoid blue. Black and blue together create a zone of indecision: from a distance it reads as one muddy dark, up close as two different blacks. It has been the most common mistake of recent years.
Blue dial
Blue is the most demanding dial. It already carries a strong colour, so the strap has to decide whether to support it or stay quiet.
The safe route is brown. Blue and brown have gone together forever — they are complementary in practice, not only on the wheel — and a warm brown lifts the blue without competing with it. Cognac on deep blue is one of those pairings that always works, even across very different watches.
The tone-on-tone route demands precision: either the strap blue is the dial blue exactly, or you move to a clearly darker blue, close to midnight. Nothing in between.
Black on blue is the cautious choice, and a slightly sad one. It never fails, and it never adds anything.
Silver or white dial
Here the strap becomes the only colour on the watch, which is when it weighs most. A silver dial with steel hands and indices is entirely neutral: whatever you put underneath becomes the personality of the watch.
Watch out for a difference that looks small and is not. White is cold, silver is not — silver has a reflection that turns warm or cold depending on light and finish. A sunburst silver dial turns warm, a matte one stays cold. Whether brown works or clashes depends on this, and you can only settle it by looking at the watch in daylight.
On pure white I recommend darks: black, dark chocolate, midnight blue. Contrast is the point of that dial. On warm silver, the whole brown family opens up.
Green dial
Green entered the watch wardrobe only a few years ago and carries one rule: do not put another green next to it. The risk of two disagreeing greens is higher than with any other colour, because green has an enormous range of undertones.
Brown works — every brown, from cognac to dark chocolate — and so does black. Beige and sand work if the green is pale and matte. Blue does not.
Aged and vintage dials
Once a dial has taken on patina, the colour to answer is no longer the factory one but the one it has become. A black dial turned warm brown — the tropical dial — asks for brown leather, not black. Indices that have gone from white to cream ask for a strap carrying a little of that cream, even if only in the thread.
It is the one case where I suggest choosing the thread before the leather. An ecru thread on brown leather picks up the cream of the indices and holds the whole watch together. A detail a few millimetres wide that reads from half a metre away.
The metal, which decides more than the colour
Before the dial colour I look at the metal of the hands and indices, because that sets the temperature. Gilt indices on a black dial make that black warm, and brown comes in. Rhodium-plated steel keeps it cold, and brown struggles.
It is why two watches with the same black dial can ask for opposite straps.
What I do not know
On stone dials — meteorite, mother of pearl, lapis — I have no rule. They shift colour with the viewing angle, and every example differs from the next even within the same model. There the only method I know is to hold the leather beside the watch, in daylight, and look.
The short version
Repeat a colour that is already there, or step away completely: never get close. Look at the metal of the indices before the colour of the dial, because that is what tells you whether the watch is warm or cold. And judge in daylight, never under a lamp.
If your dial does not fall into any of these cases, tell me the model and the colour of the hands: it narrows down to two options straight away.