Actually this gives two images representing one image twice one of which is darker than the other. The comma "," just a separates two data sets. You can skip "," and leave blank space that gives the same output. Or if you use ";" then the two image will be displayed one above another in a vertical form.
In order to get or see one image, just to use:
x1 = (cb − bmean); x2= (cr − rmean); subplot(211), imshow(x1); subplot(212);imshow(x2)