MatLab Code For Converting RGB Image Into 4 shaded colors

This Is My Original Picture...

And Following is the MatLab Code....

a = imread('mirwise.jpg');
a(1:640,1:360,[1 2])=0;
a(640:1280,1:360,[2 3])=0;
a(640:1280,360:720,[1 3]=0;
a(640:1280,360:720,[1 3]=0;
a(640:1280,360:720,[1 3])=0;
imshow(a)

This Is My Converted Picture.