FAQ: "Part of the directions says distilled water and other steps say DI water…which should be used?"
When you measure the color of transparent liquids, it is a typical practice to standardize in transmission on the cell filled with a clear, colorless solvent to negate the effects of the cell and solvent from the measurement. This top-of-scale procedure, also called "standardization on a blank", sets the cell and solvent to 100% transmission, leaving just the color of the transparent liquid sample as the measurement.
For this test, any reagent grade water that serves as a colorless, aqueous solvent will do - distilled water, deionized water, HLPC water - all will work. Just don't use tap water.
A good PQ (Performance Qualification) step after standardization to verify that you are set up correctly is to read back the cell + solvent as a standard or sample. It should always read L* = 100, a* = 0, b* = 0, Y Transmission = 100, Yellowness Index = 0.