For your first house i'd recommend you stick to the basic way of building houses. Use Graal's "Predefined Objects" for houses (Wall, Corner, Floor, etc.) Focus on the way tiles line up with each other and play with it until you have a perfectly built, normal looking house. Anyone could have built it, but the point is that you did... that brings you one step closer to being a good level builder.
This is what your house would look like if you built it with the standard tiling:
As you get more practice, you can experiment with doing more creative stuff. I do like the "homy" feel you created with the brown wall tiles. Changing the color of the level IS something an experienced designer might want to do. But I think the propper way to do it would be to use a custom tileset with custom colors. That falls into graphics art. (basically you'd need to make a new version of pics1.png and change the colors of the tiles in photoshop). Here's an example of how to achieve the same color effect by editing the tileset.
Keep designing man. But start with the basics and let creativity come in time.