One meeting with my dear E. later, and I've got the color scheme for the empty room! :D
The bookshelves and the desk are a natural wood color. The old couch and armchair will be covered with dark chocolate-brown throws. There will be off-white and fern green cushions.
So, dark brown, off-white and green. And, I have an idea! I could even add in bright blue or turquoise accents! Kind of like in this Japanese paper pattern:
Verrrry prrrretty!!! <3