Colorimetry, one of the central themes in contemporary chemistry, generally relies on spectrometers to quantify specimens of interest. Developed herein is a rapid screening scheme to determine whether the amount of an analyte falls into a diagnostic concentration range by the naked eye. This is particularly important for applications under circumstances where instruments are not readily available. The aforementioned goal is demonstrated by utilizing copper-thiol chemistry as a model system in which polymerization occurs within a certain range of copper-to-cysteine mole ratios; hence, the targeted range of copper concentration is tunable by adjusting the amount of cysteine. Within and outside the range, the solutions appear cloudy and homogeneous, respectively. The reaction mechanism is proposed and scrutinized. The detection scheme is applied successfully on samples of human serum (15.7-23.6 μM) and pond water (<3.0 ppm or 47 μM) with a handy laser pointer.