1. A non-transitory computer-readable storage medium having stored therein an image processing program executable by a computer of an image processing apparatus, which is connected to a real camera for capturing an image of a real space and a display device including a screen on which the real space is displayed, to cause the image processing apparatus to perform operations, comprising: obtaining a real image showing the real space that is captured by the real camera; detecting a specific target object within the real image whose color is known; obtaining color information about at least one pixel of the specific target object the real image, wherein obtained information includes color information about at least one pixel of a portion of the specific target object that has a higher brightness than that of other portions of the specific target object; setting, based on the color information obtained, at least one light source parameter relating to a color and/or a brightness of a virtual light source set in a virtual space; generating a virtual object image that is obtained when an image of a virtual object in the virtual space, which virtual object is illuminated by the virtual light source whose at least one light source parameter is set, is captured by the virtual camera; and displaying the virtual object image in a manner such that the virtual object image is superimposed on the real space displayed on the screen.