Marley about covered it!
I've got a lot of bags and do change frequently, though every season I have a favorite "go to" bag that tends to get the most play. This season I have two go-to bags, both are cognac, one is a Coach shoulder bag and the other is a Clare Vivier fold over clutch. (I'm asking for another C. Vivier clutch for xmas -- red -- I really love the size and the leather).
When choosing a bag, first I consider function & size. Do I need room to stash scarf, hat, gloves, etc... am I traveling and a clutch is more practical because I can tuck it inside my carry on bag.... is my back hurting and a clutch will ease it... do I want to be hands-free and take the cross body.
Next I consider the "vibe" of the outfit and what works best for outfit/occasion: is it the weekend and I want something slouchy and more casual, is it work and I want something more crisp & polished & structured, am I going out for the night and want something with some glitz?
Then I consider color and texture. I do like to give my outfits a pop of color, especially because I do wear quite a bit of black & neutrals. But bag choice also relates to my footwear: if I'm wearing solid on my feet then I want a texture on my bag, or vice versa.
Lastly I stand in front of the mirror and take in the overall effect. Does the bag work or is it competing? Does it work with the scarf I plan to wear? What about my coat? This is how I wind up with so many bags -- because I'll often think "it's okay but would be even BETTER if I had a bag like... "
When I'm happy then and only then do I do the purse swap. I used to have one of those organizers but I find it's actually quicker to just swap, while also cleaning out tissues/receipts/coins etc.