Here are some sketches I just found after flip flopping through some sketchbooks of mine.  For the acrylic city painting, one thing I realized very quickly is how often you have to change the colors when you're painting the sun going down.  Without realizing it, I kept looking up and then down at my sketchbook saying "no, that's so much darker than that. what am I thinking?"  As I kept going, the painting progressively got darker until I realized it's because it was actually getting darker outside.  I figured that was a good place to stop, so my painting wouldn't become a big rectangle of darkness.