How can i extract green rectangle area from the rgb image ?

I have two image. one i take in night and another take in daylight. So i only mask the region where green pixels exists. but problem is that for every image i need to change the value. how can i fix it the same value for all images.

Im = imread('4.jpg');
figure,imshow(Im);

lab = rgb2lab(Im);
figure,imshow(Im);

% Im = imresize(Im, [512 512]);
%  figure,imshow(lab(:,:,2));

   I1=lab(:,:,2);
    I2=lab(:,:,3);
    I3=lab(:,:,1);

   mask1=I1>-50 & I1<0 ;
   mask2=I2>30 & I2<50;

   mask =I3 & mask1 & mask2;

%  mask=mask1 ;

   figure,imshow(mask);

     diff_im = im2bw(mask,0.18);
      figure,imshow(diff_im);

Answer 1

[Image Analyst]

Use the Color Thresholder on the Apps tab.