how to differentiate the same color objects with different intensity?

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hi, In the given image, There are 3 leather pieces of brown color and among them one is dark brown in color[the rectangular smaller piece with high intensity].I have to highlight that odd piece(dark brown piece).The procedure i followed in my code is 1.converted given image to HSV 2.checked the saturation range(S value) as well as the brightness range (V value) of the odd piece and other two light intensity pieces. But the problem is ,The odd piece values(both s and v value ranges) overlaps with the the other two pieces(light intensity) values.so,which color model best suits for this problem? if illumination changes the values get again changed,how to tackle this problem?what type of camera should i have to use?.please help me asap