Illumination light color estimating method of obtaining white balance and image sensing apparatus

When average color data indicates blue, the blue component is reduced from the average color data. A line is drawn diagonally from that point downward to the left at 45° on the coordinate system. The intersection between this line and the abscissa is estimated as the color of the light source. When average color data indicates green close to cyan, the cyan component is reduced from the average color data. A broken line is drawn diagonally from that point upward to the left at 45° on the coordinate system, and the intersection between this broken line and the abscissa is obtained. In this case, the value of the abscissa of estimated light source color is set to 0.