First, what do you need light for? Do you do paperwork? Is your desk in your window? You'll be getting afternoon sun. I'm recommending LED lighting to people, but Fred says it has a way to go in brightness and affordability. I recommend sky tubes to homeowners. I've had 2 in one of my houses.

First, get paint chips or some other sample of the colors of the draperies and the floor. Take these to the paint store to help select possible colors. I suggest that after you narrow down your choices that you buy samples of the colors that you are considering and paint them on foam board to get an idea of how it all will look in the room. For a minimalist look, you might consider a the color of the draperies but toned down so that it is almost white.

White curtains or very pale grey will keep the light levels high in your room and make a nice backdrop to your furnishings. I think patterned curtains are more casual and with the amount of windows you have, they would make a huge statement in the room. Darker curtains will reduce the light and can make the room look dingy. If you felt you don't want perfectly plain fabric, look for a slub fabric or something with some interest or texture in the weave. You could even have a self stripe which can look very elegant. I have just finished new curtains through my house and did the double track where I could and used a thermal lining that I can pull across to keep the cold out and it worked very well and is also working well now in the heat keeping the rooms cooler - in fact it is amazing some days the amount of heat trapped between the curtains and the windows and kept out of the room. In my bedroom I used a heavy sheer curtain to be able to enjoy the view of the garden while still maintaining privacy and the thermal lining gives me full privacy at night. I'm thinking this could be nice for your room when you have such a lovely outlook too and you do need something to keep the sun off your furniture and furnishings. There are also thermal roller blinds you could use instead of curtain linings, these do darken the room but they work very well. If you use the linings though, there are different weights, I used one pass in my bedroom and other rooms, and it does not darken the room very much at all. In my lounge room where I really needed heat blockout in summer, it was a bit of trial and error - I used triple pass and it does significantly darken the room to the point of totally gloomy - I really don't think I would use it again.

What a lovely room, my thought are: - agree that the couches should be rearranged, away from the largest window to let in more light - if you are buying new couches, I'd go for a lighter colour (if you don't have young kids) and some cushions perhaps with a pop of colour - coffee table could be rectangle or round - either glass or a light colour timber - some wall art, floor standing lamps, mirror and indoor plants would also help to brighten up the space - roller blinds would be my choice as I like that they let in a lot of light, but if you prefer curtains then I'd go for a light neutral colour (white/cream) Hope that helps :)