Wrong sofa colour choice, what would you suggest for walls?

[icanneverremember]

Hi all,

I bought a sofa, stupidly, without getting a swatch first. The colour is quite a cool, bluey grey. I now want to paint the walls but really don't know what colour to go for. Our colour scheme round our house is grey, green and blue, in warm pastel hues. The room is a play room and north facing - what colour should I do on the walls??