I use to be in the graphics and printing industry, and matching colors was crucial.Still can't see anything on the Salifert test but Hanna says my PO4 is at 0.25 and NO3 is at 49.6.
I can get close with the Salifert compared to Hanna, but man it's tough for me to see it sometimes on the PO4.
Try putting a test cup of plain water next to your sample. See if you can see any difference. If not, maybe you got a bad test kit.