[phil]

I thought about this for a while.

I'm not an expert in oil painting technique, but I think one possibility is, you tend to overpaint an area at a time. So: mix some paint, layer up part of the canvas. Come back later, mix some paint, do it again.

If that's how it works it would explain a bunch of things in the comparison images. Like: how come the bridge is yellow, but its reflection in the river is bright green? Maybe they were painted at different times.