Like a coloured waistcoat, it functions best when peaking out around the jacket’s waist button, as shading and outline. Without the jacket it is a dominant colour rather than an accent. Yet it is also subtler and more practical than either of the silk accessories mentioned above, and benefits as such. It is harder to appear foppish or affected, smacking as it does of utilitarian warmth.
The sweater can be V-neck, cardigan, shawl collar or anything else – the only thing to bear in mind is that the neckline should roughly mirror that of the jacket it is worn with. A loosely buttoned cardigan will seem more natural with a one or two-buttoned suit. A round-neck sweater is always going to suit a high-fastening three-button suit.
So to the colours. I have three recommendations that particularly please. They are bottle green with navy, biscuit with brown and red with green (probably tweed).
A dark blue such as navy has essentially two options with its accent colours: very subtle or very brash. Red socks with a navy suit work somehow because they are over the top, equally yellow and other bright colours. But the most harmonious accent colours are dark, strong colours, principally green and purple (see picture from the Sartorialist).
So with a navy suit try a bottle-green cardigan, loosely framing the jacket’s lapel, enriching the blue itself in its richness and reflection of tone. Perhaps a blue/white striped shirt, and a dark tie if desired.
Second, bringing out the browns of a chocolate-coloured suit through a biscuit colour, or tan. The pleasure of a chocolate-coloured suit is that, despite its unusual colouring for business, it is so dark most people take it for grey. The addition of a sweater in either a pale biscuit colour (think Rich Tea) or tan (think highly-polished Oxfords) brings out that rich brown tone. Again, I’d go for a blue or blue-striped shirt (depending on the balance of pattern elsewhere in the outfit).
And finally, bounce up a green tweed with something bright – and nothing’s stronger than red. Strong colours work so well with tweed because it is so subtle on its own, quietly hiding flecks of autumnal colours in its thick weave. The country squire’s socks of bright red and bright yellow work well for this reason. As a sweater against the cold, perhaps over a pale yellow or cream shirt, it is no longer so affected as the socks, but charming and ever-so practical.