How to implement LAB color space to detect yellow in image?

>>Hello, I need some help to detect yellow exudates in a retinal image. LAB or HSV colour space can be used. I have chosen the LAB colour space. How must the if statement look if the user use gtool and clicks on yellow dot to segment that or identify that as a yellow exudate?

here is my code:

   RGB2=im2double(OriginalRGB);

   cform = makecform('srgb2lab');

   lab = applycform(RGB2,cform);

figure,imshow(RGB2);

[d1 d2]=size(RGB2);

   yellow_redThreshold   = 200;

   yellow_greenThreshold = 200;

[X Y]=ginput(2);

   x=round(X);

   y=round(Y);

   punt=[x y]; 

   for a=1:x

     for b=1:y    

   if (yellow_redThreshold < punt <yellow_greenThreshold)

       figure,imshow(punt, []);

      title('Yellow exudate');

   end

      end

I want it to do something like this

[Narges M]

Do you mean something like this?

punt=lab(x,y,:); 

if ((punt(1,2,1) < greenThreshold) && (punt(1,3,1) > redThreshold))
...
end