I wonder what tolerance is on the pressure ?
There is no stated tolerance on the packages. This can be a bad sign, so I bothered to put the caps on a few times as I approached the 37 psi I like for the rear tire and and 34 psi for the front. For the 2 devices I used, (each package has 4), the pressure just at full green was within 1 PSI of the labeled value, according to my pressure gauge (which also has unstated accuracy but appears to agree with a fancier one I do not carry on the bike.) I could not bother to determine accuracy better than that, given I do not use these devices to set the pressure, but merely to avoid riding on truly underinflated tires. Of course, other devices may be further off, so I plan to "calibrate" whenever I have to replace one.
On the package are the claims
"Full Green: Indicates pressure is ## PSI"
"Half Green: Indicates pressure is <= 5 PSI Low"
"Clear: Indicates pressure is <= 10 PSI Low"
(where '##' is either '32' or '36')
So, it's an analog indication. With a little skill at interpolation, it is not unreasonable to expect to perceive a 2 PSI drop if pressure is at the full/less threshold.