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In order to be color independant, do not use color information to detect your ball. This is exactly what u doing, by thresholding by certain values, which i can deduce from your answers.

Just create a grayscale image, apply edge detection, then use the circular hough transform to detect blob/circle structures. By then giving some thresholds on the size of the actually object, which you know since its a tennisball, the segmentation should be quite obvious.