Differences in sun exposure. Of course most leaves change color as the days get shorter and they receive less and less sunlight for photosynthesis. If you watch a tree, the lower leaves begin to change first, because they get less light than the leaves at the top. Then slowly the color creeps up the tree.

The same thing is happening with the vines but from corner to edge instead of top to bottom. The vines near the edge of the buttress receive more sunlight than the ones on the left, which are in the corner. Shadows are cast from the buttress they are climbing on, and there are probably shadows from other obstructions too–other buttresses, other buildings, maybe a door propped open.

Hey, I keep making a joke about B.O. conspiracy theorists, and it wont post. filter?

Wow! Trippy!

Can I have some of what you’re smoking?

My (uneducated) theory is that it may also have something to do with temperature differential as well; there’s a tree behind my house where the south- and east- facing leaves have started changing before the north- and west- facing ones, where the tree is almost completely shaded by the house across the alley.