... try working with a layer with layer mode set to burn or similar. I found that it can produce pretty nice effects for something like wood. Play with layer modes to find the right one.
But... but... but, that's, well, ah, not
pixel art, is it?
109 colors in version #3, >256 in #4...
I'm sure the experienced artist can use layer modes in this way to save time
and knows how to retain control, but I wouldn't recommend this as a learning tool.
The point in doing it by hand is, mostly, to learn how things
look like, more than how to actually draw them. If you let a tool do the drawing for you, you skip a lesson.
This house is way too big in my opinion, esp if you plan a pixel city.
I generally agree about comments about shading, contrast and detail.
The contrast difference btw the 2 walls is almost non-existent, compared to the roof. Also, the texture should be much more contrasted and apparent on the sunlit side, than on the shadow side.
Think about shifting hues on both wall and roof: shadows will tend to be a different color than highlights, depending on the situation (bluer shadows on a sunlit day outside, for instance).