inverting the background and object colors

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I read in documentation that:

In OpenCV, finding contours is like finding white object from black background. So remember, object to be found should be white and background should be black.

In my case, all the images I am dealing with are colorful but their background is always white. Is there any solution in OpenCV to make the background of these images black and the objects white in order to apply cv2.findCountours() function ?