A natural color on the walls of your cozy apartment could work nicely but I would suggest keeping the colors of a similar value rather than having lots of contrast.
I love the color scheme in the room below from the House Beautiful website. I think your room would work well in this color scheme and give you the "grown up" look you are after. It has the following Sherwin Willaims colors:
SW 6400 Lucent Yellow for the walls
SW 6399 Chamomile for the trim
SW 6163 Grassland for the ceiling

I elaborated a bit more on why the elements in the example room help to expand the space on the Color For Your Home blog... |