Changing color within a specific area with opencv using c++

Question

I would like to know if there's a simple way to change a color, say from a shade of grey to white, within an area of interest that would be determined from the image frame on the outside and a contour, returned from the opencv contour function. I mean, let's say I have an image with a black background and 2 circles, one inside the other, between the circles the color is a shade of grey, and inside the inner one is black. I found the outside circle contour, now I want to change the outside black for white.