Most outfit advice is about clothes. Sunglasses are different: they are small, but they sit in the middle of the picture, right where people look first. That makes the colour of the frame work like a punctuation mark. It can end the outfit with a firm full stop, add a warm comma, or glint like an exclamation.
This piece compares the three frame colours you will meet most often: black, tortoiseshell (a mottled brown and amber pattern) and gold-tone metal. It gives you one simple rule for matching each to what you are wearing, and a small tool to try it. The lens colour is a separate choice, covered in its own piece on this site, so here we only look at the frame.
Black is the sharp one. A black frame draws a clear outline around the eye area, which makes any outfit look a little more graphic and a little more decided. It is easy to wear with black, white, grey and navy, and it is the safest choice if your wardrobe is mostly cool or neutral colours. The trade-off is that it is also the most noticeable frame against light, soft colours such as cream and camel.
Tortoiseshell is the warm one. The pattern is a mix of browns and ambers, so it echoes leather shoes, a belt, a camel coat or a wooden watch strap without matching any of them exactly. It sits comfortably with denim, olive, khaki and cream. The one place it can look slightly out of step is a strictly black-and-white outfit, where it becomes the only warm thing in the picture. That can be a nice accent, or it can feel like it belongs to a different outfit.
Gold-tone metal is the shiny one. Thin metal frames take up very little space, so the outfit stays the main subject, while the metal adds a small point of light. They work best when something else you wear is also gold-tone: a watch, a ring, a belt buckle, a zip. When your other metals are silver, a gold frame can look like a mismatch.
Match one thing, not everything: a colour or a metal.
Choose one of three frames and one of four outfits. The drawing shows the frame next to the outfit's three main colours and its metal (the buckle or watch). The marks below the drawing show whether the frame repeats an outfit colour, and whether it repeats the metal. This is a rule of thumb, so the note under each result also says when to break it.
The rule behind the tool is short. A frame looks considered when it repeats one thing from the outfit: a colour (black frame with black boots, tortoiseshell with a camel coat) or a metal (gold-tone frame with a gold-tone watch). It does not need to repeat both, and it does not need to match everything. If it matches nothing, it can still look good, but then it is an accent, and an accent should be the only one.
Break the rule on purpose. A black frame on a camel coat is a strong, graphic choice. A gold frame with silver jewellery is fine if you wear a second gold piece to show it was meant. The rule is there so that the odd choices are decisions rather than accidents.
Also look at what you already own before you buy. Your most-worn bag, belt and shoes tell you whether your wardrobe leans warm or cool, and the frame colour that echoes them will be the one you reach for most.
A colour that goes all the way through. In a good frame the colour sits in the material, so a scratch does not show a different colour underneath. Tortoiseshell patterns should look layered, not printed on the surface.
A metal that matches the rest of your metals. Hold the frame next to your watch, belt buckle or jewellery. If they are all the same tone, the look hangs together.
A frame weight that suits the outfit. Thin frames leave the outfit as the main subject; thick ones become the main subject. The weight of the frame is covered in a separate piece on this site.
A real UV rating. The colour is for style, but the lenses are there to protect your eyes. Look for a UV400 label or the CE mark, and treat a very cheap pair with no label with caution.