I heard somewhere that you should use a light colored rosin in the summer and this weekend I learned why. I was playing an outdoor gig, fiddling away with my music partner (harmonica and guitar). I couldn't find my regular rosin but I found a piece of really dark rosin in an old fiddle case so I took it with.
After some fiddling I got out my rosin and stroked my bow over it. I have gut core strings and I was already having trouble keeping them in tune in the constantly changing humidity and temperature. After using the rosin it felt like the strings were 'resisting' me somehow. And the sound got rougher and more coarse until it wasn't even music. And the rosin seemed to turn into GLUE on the strings, heavy and sticky.
I spent a lot of time wiping the strings off and when I got back home I cleaned the strings and the bow and finally found my light rosin.