Hi Tim,
Try replacing "plot" with "imshow". Also need to remove the quotes from the second command in the last line...
subplot(2,2,1);plot(RGB);title('RGB')
subplot(2,2,2);imshow(RGB1);title('RGB1')
subplot(2,2,3);imshow(RGB2);title('RGB2')
subplot(2,2,4);imshow(RGB3);title('RGB3')