Hey man, good luck with your grow. I've bought a couple of ph testers. My first was just litmus paper. The calibration tests I did showed great results. Changed colour well with bleach and vinegar. But when testing my water, the results were difficult to determine. Not wanting to waste any more time and money on other like types I bought a digital one. Only cost me £10 from amazon. Cheap and does the job. Fairly accurate and had a much better experience. I suggest you do this rather than rely on colour charts. They can be unreliable ✌️