Is there really a tradition of some kind to eat white bread when it's hot outside and dark bread when it's cold?
Short answer: No.
There does not appear to be any such tradition.
I got curious and googled around a bit, and couldn't find anything.
That, and being from a country that could easily get to -15c in winter and 25c in summer, so if anyone would have such a tradition, I think it might be us ;-)
Having said that, if OP knows of any places with such a tradition, it would be interesting to know...