A few years back I bought some tire pressure valve caps from Napa Auto parts that changed color when the pressure dropped by more then 4psi,
they looked alot like these and measure PSI of 26, 28, 30, 32, 34, 36, 38, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115 and 120
The ones I had were of another brand but I never had issues with them, they were 50PSI and warned me a number of times of slow leaks that I then repaired.