Tumblr'd here: [link]
I admit...this almost makes me wanna be a Slytherin...
This one is very different from the first two. I felt Slytherins in general would be more fond of order, hence the symmetry. And the simple color scheme isn't as bad as it seems, since you get color from the lake out the window. the lake is as much a part of the room's decor as the room itself
Edit: I feel the need to point out that these are out of my head, not based on the descriptions provided on Pottermore. Just in case people are wondering why I'm right on some counts and not on others
Edit 2: You guys, I am sooo overwhelmed by the positive response these have gotten! Thank you so so SO much~!! Also, I've gotten quite a few people asking if they can use these as a reference for their fanfics or own artwork, and the answer is YES! All i ask is that you be sure to credit me and if possible link back to these pieces. And tell me if you use them, because it makes me super happy knowing people have found my random designs useful