When I use my K-1000 to run OTO and pH tests, a funny thing happens. I close one eye and look through the pH tube; around 7.5. I look at the OTO tube and try to figure out what it says. Then I look back at the pH tube; now it looks more like 7.6 or 7.7. (If I switch eyes then the pH color again matches 7.5.)
This seems to be an effect of "retinal fatigue" where staring at one color will cause the retina to become desensitized to that color for a while. So, after staring at the OTO tube, part of the retina becomes less sensitive to yellow, then the orange in the pH tube will look more reddish and appear to read higher.
So be careful when you're doing color matching! Look at the pH tube first, or switch eyes after looking at the yellow OTO tube.