actually, if i can guess, i would say THU had it easy due to my laziness. i am using only the trackpad on my powerbook, so my "sketching" can get pretty wiggly (notice the shape of the windows). as a result, i made the buildings with nice easy (for me, and for neighboring artists!) straight lines.
further, due again to my lack of creativity, i filled everything (including the shadows) with solid colors. very easy to sample with the eye dropper. actually, this was partly a conscious decision (and only partly lazy!). very seldom do i really want to challenge the neighboring artist with something like a texture-fill or gradient, and if i do, i try to leave a chunk of the pattern to allow rubber-stamping.
maybe i am too nice? then again, okay, maybe it was laziness only! haha