Flannel trousers have been a menswear staple for at least a century. In the first half of the twentieth century, they were worn mainly in casual settings and even for playing sports. These days, they are extremely useful as part of smart casual wardrobe for  the colder seasons. Whilst they are a level below a worsted wool suit in terms of formality, they still offer a look that is more than smart enough for most offices and professional settings. 

The most useful colour is going to be grey for flannel trousers. This is because it pairs well with the major sports jacket colours of blue, brown and green. Within grey, all of the different shades are useful options. Mid-grey is most versatile but charcoal and light grey will also find lots of potential uses.