How to insert a shape on an gray image

yellow = uint8([255 255 0]);

shapeInserter = vision.ShapeInserter('Shape','Circles','BorderColor','Custom','CustomBorderColor',yellow);

I = imread('cameraman.tif');

circles = int32([30 30 20; 80 80 25]);

RGB = repmat(I,[1,1,3]); % convert I to an RGB image

J = step(shapeInserter, RGB, circles);
imshow(J);