Could be any color.

White or ivory

White or ivory is standard, but I often use a putty or khaki color if the exterior of the home is stucco or tan. The putty/khaki lining blends better with the exterior. However, sometimes neighborhood deed restrictions limit the color showing to white or ivory.

Absolutely white or ivory. Same color should be on all windows. It is a personal pet peeve when there are all different colored window treatments facing out.