This. Seasonality is way more about weight and texture than color. Brown. All. The. Time.
I agree for the most part, but I'm going to use what you said as a spring board to get a little philosophical. I would add that while almost all colors are good for almost all seasons, some tones and shades certainly suit some seasons more than others. This isn't because of some English custom traceable back to Victorian royalty, but because of the varying amount of natural light that accompanies each season. A good rule of thumb which most people understand intuitively, but someone with are large wardrobe and a discerning palette can intentionally put to good use is that lighter tones look better in more light and darker tones in less light.
If colors themselves are at all seasonal I would guess its because different places, for a thousand and one reasons, have different natural color schemes and an attempt to match these color schemes is aesthetically pleasing. This is the theory behind the dictum that the greys and navys suit cityscapes better than hunter greens or certain varieties of brown. However notice that the grey and navy rule has many exceptions. There are many places within many cities where grey and navy are not the colosr of the side walks or the buildings or the vehicles or the common styles of interior decoration. In many places brown and various shades of tan look very at home, think pale sand colored buildings common in SoCal or wooden interiors, as opposed to cubicle municipalities. Seasons, like cities have various stereotypical colors, light blues in spring, black in winter, rusts in fall, etc. Yet, also like cities, the seasonal colors vary from place to place. Here in LA I don't get as many rustic colors in the fall as I will when I'm in Scotland next fall. Seasonal rules hold a grain of truth but they really all reduce to a joint awareness of one's surroundings and some basic principles of aesthetics.
In one sense this can be infuriating, because it means that there are no hard and fast rules for building a wardrobe that will work for everybody. On the other hand with time and effort the SF clothes hobby can take on a whole new dimension as a subtle form of local patriotism. Instead of debating what is seasonal and what isn't seasonal it would more profitable to discuss what specific considerations make X suit the environment in which it is worn.
Also, @Claghorn, just so you're aware, I'm a fan of your understated, but far from soporific fits. You do a good job of combining the few authentic insights of the igent color palette with the conservative elegance that the old guard used to preach.