I think that having an accent wall painted either your favorite color or covered with a fun wallpaper would be cool. Then you could make the other walls white with some nice shelves to display all your colorful collectible figures. That will make your collectibles stand out really well.
One tip if you want the room to feel bigger, is to paint it a lighter color. Light colors make a room seem bigger, while darker colors make it seem smaller.
I know this is 3 months late, but if you ever want to maybe redecorate your room again, try a mood board. I’ve been using this website-http://www.gomoodboard.com/boards/bXrzem_9/edit – to get ideas for rooms. Mr. Kate always uses mood boards when she designs.
GreenĀ combines well with golden-brown, orange, salad green, yellow, brown, gray, cream, black, creamy-white. I think it would also go well with the minimalistic palette you suggested.
Try hanging your curtain rod higher than the actual top of your window. This will giveĀ an illusion of added height AND width. You should also try placing smaller or lower near or under the window so the eye is tricked by scale. My last tip for you is to try curtains with patterns. Vertical patterns will make the window seem higher, and horizontal patterns will make it seem wider. Good luck!