>> im = imread('ny.png');

>> r = im(:,:,1);

>> g = im(:,:,2);

>> b = im(:,:,3);

>> imshow(r), imshow(g),imshow(b);

分别显示有颜色的

>> r1 = cat(3, r, zeros(size(r)), zeros(size(r)));

>> g1 = cat(3, zeros(size(g)), g, zeros(size(g)));

>> b1 = cat(3, zeros(size(b)), zeros(size(b)), b);