To assign a different intensity to an object present in an RGB image, basically there are two steps:
(1) To identify the object and positions of the pixels that this object sits on;
(2) Assign intensity to these pixels.
You can use Computer Vision System toolbox or Image Processing toolbox to do the first step, for example, function ‘imfindcircles’ can be used for finding circles. Now I assume you already have those target positions in hand. To change intensity of certain positions, here is a simple example:
I2 = I;