Let's say there was a five year old desperately wanting his own room (maybe he currently shares with his 2 year old brother!) I didn't want a "one theme" room but to have touches of his current likes. Things like pillows, sheets set, and art that could be changed throughout the years as interests change.
I'd start with a cool light blue for the walls then float shelves (white) to display current art work, toys, and knick-knacks. I'd add a bright fun parsons desk and white desk chair for those intense coloring sessions and little Lego building. A DIYed striped rug (I've been wanting to try this for a while) and bedding with layered pattern and color. Some funky accents like a cardboard mounted deer, pillows, and a large kid-like map of the world. He'd be one lucky little guy!