The seasons hacks the routine that loads the pallet from the tileset, so that it checks the time, and loads a different pallet depending on it. You can see it here:
http://romhacktube.com/video/MMSWS7MA49U3/ASM-Hack-Seasons-in-Fire-Red
Meanwhile, I'm also hacking the worldmap pallet, block loading, and behavior loading. So, if I wanted, I could make small ponds freeze in winter, so you can skate across :)