how to change specific colors of image?

clear all;
RGB = imread('im5.jpg');

f1 = figure(); imshow(RGB);

[r c z] = size(RGB);
RGB = RGB(floor(r*0.2):r,:,:);
[r c z] = size(RGB);
I = zeros(r,c);

for i=1:r
    for j=1:c
        if((RGB(i,j,1)>100 && RGB(i,j,1)<250)&&(RGB(i,j,2)>50 && RGB(i,j,2)<180)&&(RGB(i,j,3)>0 && RGB(i,j,3)<100))
             I(i,j)=1;
        end
    end
end


this is a code which read image pixel wise, and changes yellow pixels to white and rest to black. I want to change color of all pixels to black except white. White should remain as white and rest all pixels should change to black.
Can somebody plz help me?