As it happens, I have a bag for all seasons, but since you read the beginning of this post, you know that it's not so simple. My bag for all seasons is adorable and classy, a huge step up from the "Dubble Bubble" vinyl bag from Goodwill I'd been using for years. This bag is also big.
I can fit a 5-subject spiral notebook, a hardcover book, and a bag lunch inside it, in addition to the various breath fresheners, medications, papers, gadgets, and chapsticks that accumulate in my bags. This is pretty darn awesome when I need to transport a lot of stuff around. Unfortunately, on days that I don't need to bring half of my earthly possessions along with me, the bag is just too big. I know comically oversized handbags are "in" now (I think? Are they still in?), but I'm not much for trends. And I find that carrying a big bag around just encourages me to bring along more things that I don't need.
My summer purse problem was neatly solved by my mom's purchase of a bag that she eventually deemed "too cute" for her. I was, of course, more than happy to take it off her hands. This purse is the perfect size for my day-to-day maneuvering, but it is indeed cute: artsy flowers all done up in bright sunny colors. This was great when it was tagging along to summer festivals, but I'm already starting to feel weird carrying it in the chilly late-October weather. I need a bag that can serve me through the fall and winter, just like my trusty wool peacoat.
Obviously, I respond well to red plaid for the cooler months. But in general, I like either darker colors or really rich colors (deep reds, blues, purples, greens) in autumnal/winter wear. The muted colors are popular for obvious reasons, but so are the rich ones--they make a great statement against a dark coat (and the muted landscape). When shopping, don't forget to consider, as I neglected to, the size that you really need for day-to-day use. You might want to use your current purse as a guide, or, if you're unsure, bring along the biggest item you'd want to carry in your bag with you shopping. If it fits, success! You've found yourself a bag!