how to make a particular region to appear white in color

hello I am creating a image mask of a retina image. i want a particular region to be white in color ..is it possible to do ? how can i do it? i want the region inside the ring to be white in color. to get this image i have applied morphology and canny edge detection

Answer 1

[Image Analyst]

binaryImage = imfill(binaryImage, 'holes');