Thanks for this question !
So, we have not yet tested chlorine measurements with the colorimeter, but it should be possible. The most common method for measuring chlorine colorimetrically is with DPD (N,N Diethyl-1,4 Phenylenediamine Sulfate). DPD turns a pinkish color in the presence of chlorine, which is measured at 520 nm.
The Free Chlorine DPD Power Pillows from Hach (Part # 2105569) look like a good place to start. We have used the Hach powder pillows for other colorimetric measurements. (Link to download the Hach method sheet). I had a read through of the methods and it looks pretty straight-forward. Add a powder pillow to 10 mL of water sample in a glass container and measure absorbance at 520nm.
To use this method with our colorimeter, the first step would be to create a calibration curve which you would use to measure all of your test water samples against. To do this you would also need a chlorine stock standard and the equipment to make a set of calibration solutions (pipettes, glassware, distilled water). To each of the calibration solutions you would add a powder pillow and measure absorbance at 520 nm (or near). Once you have the calibration data (which you may only need to do once) you can then measure your water samples.
I would need to look into this some more to help with making the calibration solutions, but hopefully this information can get you started.