I am using the Nitrate/Nitrite Colorimetric Assay Kit from Cayman. When I used the nitrate+nitrite determination assay with nitrate reductase on the same samples' supernatant, it turned pink. When I used the nitrite assay without the enzyme, the result was yellow. Am I doing something wrong? Do you get the same results? If so, is the OD in both cases 540 nm?