I agree, migration and trading may play a role, but I think when it comes to appearance of bean seed there just are particular colors and shapes that commonly show up. Actually, the same colors and patterns show up on beans of very different shape and size. It's easy to get focused on just the appearance of a bean seed when it is really only one of lots of traits. Beans that look exactly the same can have very different growth habits and so on.
Like Joseph's, one of my consistently and reliably productive beans are Pinto beans. Except I don't really know what a Pinto bean is. Basically, I think it is just a tan colored bean with darker mottling except lots of beans look exactly like that that they are not at all the same. My Pinto beans came from a store-bought bag that got lost in the back of the pantry for several years and instead of discarding, I planted them, and I've grown them ever since.
I don't know that environmental conditions explain the issue with the more colorful beans, I just don't have any other ideas on it. Not sure how to describe our climate either, you ask about forty years too late for that. We have four seasons, or we used to, but they aren't as clearly defined anymore. It gets hot here, 35 C isn't uncommon, and it can get hotter. It gets cold too, -25 C is possible although, thankfully not very often or for very long at a time. We have longer and more frequent periods without rain now, but it can also rain 15 cm in a couple hours so. Same in winter, not much snow anymore but it can still do it pretty dramatically. Ice storms are much more common than they used to be, as is large hail. Overall, it has become less hospitable to beans, the whole species, and I've began supplementing them some with cowpeas, (Vigna unguiculata) which seem more tolerant of conditions and more productive.