Hi Sara,
To my understanding you want to create an irregular regions in the image and modify the variable properties of that regions such as contrast, saturation or luminance.
Please follow this code in which I have created an irregular region in the image using "images.roi.AssistedFreehand()" and "draw()" function.
%%code starts here
I=imread('_DSC2845.jpg');
imshow(I);
shape = images.roi.AssistedFreehand;
draw(shape);
BW = createMask(shape);
BW(:,:,2) = BW;
BW(:,:,3) = BW(:,:,1);
I(BW) = imadjust(I(BW),[0.3 0.8],[]);
imshow(I);
%%code ends here
Basic idea is to create a binary mask for the required irregular region and then perform the modifications to the region and map it to the original image.

This is the original image on top of which the irregular region is created.

The irregular region which is created is modified by changing the contrast and intensity.