Depending on the age of the children that use the room (if they are over 10) let them do what they want. The room should reflect the personalities of the people inhabiting them. There is very little they can do so wrong that a closed door can't fix. it. ;-) My room as a kid was the best looking room in the house. I guess that is why I became a designer.

I agree with Sharon - the gray family is the right way to go. Make sure that they are "warm" toned grays.

You need light and bright. Being rented is tough. First change out the drapes with to the floor length. Cream with a horizontal stripe of brown to match or furniture. This will 1. get rid of the horrid things hanging there 2. pull the initial visual to that wall instead of the floor. Tall and high colors ( mango, aqua, yellow) decorations on your mantel with shorter cream and brown items. Then a large square table in the middle, painted distressed cream, with high color items setting on it. DIY project or Ikea. Green is the number one calming color, just don't let it put you to sleep. :)

The blue is awesome (grey is too trendy)! The reason it stands out to you is perhaps because it's the only real colour there. Bring in the blue in a window treatment, a small appliance that sits on the counter, a pretty tray or area rug. Also, it is more appealing often if there are more than 2 colours - so right now you have a white and blue colour scheme - bring in red or orange or fushia - something from the other side of the spectrum (like you did with the yellow flowers but in a more permanent way)!