You probably have some variant of a macintosh or trench, but a minimally lined, hooded raincoat can be useful in the same way: as a roomy, lightweight layer that won't interfere visually with nor add a lot of mass to whatever else you have on. As an added bonus the hood will keep your head dry. So there's that. Normally these types of coats come in beige or navy, but try to branch out a little—green and medium browns are also great autumnal colours for rainy days. Three options shown, all in coated cotton, from left to right: Frank & Oak [$125], Zara [$239], Stutterheim [$368].