A method is provided for determining a color using a CMOS image sensor. The CMOS image sensor includes an n-type substrate and a p-type epitaxy layer overlying the n-type substrate. The method includes applying a first voltage on the n-type substrate and obtaining a first output, which is associated with the first voltage. The method further includes applying a second voltage on the n-type substrate and obtaining a second output, which is associated with the second voltage. The method additionally includes applying a third voltage on the n-type substrate and obtaining a third output, which is associated with the third voltage. The method also includes providing a plurality of weighting factors and determining the color based on the plurality of weighting factors, the first output, the second output, and the third output.