What a beautiful room! You definitely need a paint color that will showcase the drama and elegance of the architecture. I agree with others that yellow would look great. I would choose a light, sun-washed yellow (Behr makes one called "Provence Creme 320A-2" that is really pretty in sunlight. Also, yellow goes well with your other colors of browns and reds. Stay away from greens because they will really pull out the yellow in the wood.

If you like bold colors, I think something in the muted red family would look great. Look up Behrs "Freckles" or "Pottery Red 160D-6". One of those with cream-colored/ivory light-weight curtains would look sensational with the existing wood tone.

I agree with the other commenters that the curtains should go from floor to ceiling. Also, I would extend the rod all the way to the wall so that when the curtains are opened, you can see the full width of the window.

What color is your furniture? It is difficult to see in the photo.

My suggestions would be to hange the curtains fom ceiling to floor, but consider a brighter color as the purple is not helping to bring in the natural light. Then I would address the wall...looks bare...perhaps with family pictures in similar fames to create a uniformed look or a nice reasonable priced entertainment center.

As for the curtains I'd hang them floor to ceiling buy two panels for thickness and then pull them both back to each side. Longer panels can be $$$ but IKEA carries longer lengths at affordable prices.

My thought would be to stay away from cool colors blues some greens and purples as the cool tones would make the oak more yellow. My first thought seeing your room would be gray a dark taupe grey, a warm grey. But any warm brown, golden yellow etc will help make the oak look white and not the yellow white the white walls make it look like now.You mentioned not wanting to stain the wood have you thought of painting? I'd paint not only the walls but the crown and the trim and the wood and the ceiling trim, all in the color you choose, although traditional white for the wood trim would also be pretty.