Actually the PS3 changes it's background color each month. During daytime hours when the sun is at it's highest, the color scheme is at its brightest. As the sun goes down (this is controlled by the time and timezone settings you set) the color in the background dims till it all goes black.
this is just an added effect to make the menu system look cool and more interactive. It is completely normal and nothing to worry about. Mine is currently Blue during the daytime.