You dont. The colour is determined by the time/date. The main colour changes slowly over the course of the month, and each month starts a new colour, and the whole thing goes black at night time Check your time/date settings, they are probably different (unless he has added a wallpaper of course)
There is no option yet to change the background XMB colour. You can however use your own images as background wallpaper. The only way to change the XMB background is by changing the date on the PS3 but then you would have the wrong date stamp for all the files that use it on the PS3.
I had my time zone set to Hawaii for a while as this gave me some colourful backgrounds in the evenings. Have set back to GMT now as I kind of like black, but maybe that's just me being boring...