There's nothing better expect actual machines because the pH indicator used, Brothymol blue, is the best for pHs from 6.0 to 7.6. I find that they look most different when viewed with your test solution. Then again, it could be a misprint: I SWEAR the 10ppm and 20ppm are the same color on my Nitrates card.
My Golden Rule of planted tanks: WWTAD-
"What would Takashi Amano do?"
RAOK Club #69