Are you looking for somthing like this
im = im2double(imread('image.png')); % this is the name of your image file
im_gray = rgb2gray(im);
mask = bwareaopen(im_gray < 0.2, 10); % 0.2 threshold was chosen after manually tuning the value
in_mask = 1-mask;
r = in_mask.*im_gray; % make the background value 0 for red channel
g = in_mask.*im_gray + mask; % make the background value 0 for green channel
b = in_mask.*im_gray; % make the background value 0 for blue channel
im_new = cat(3,r,g,b); % create the rgb image
imshow(im_new);
