The yellow ones apparently were for rectal use, white were oral. So you could genuinely tell them apart.
I still have one from my mom, who was an RN before the days of alcohol thermometers. I don’t use it rectally, but I have washed it thoroughly many times.
“This one’s for your mouth, this one’s for your ear, and this one goes up your butt. No, no wait, hang on. Uhh, okay, this one goes up your butt.”
The yellow ones apparently were for rectal use, white were oral. So you could genuinely tell them apart.
I still have one from my mom, who was an RN before the days of alcohol thermometers. I don’t use it rectally, but I have washed it thoroughly many times.
Now we use red and blue. There may be another color floating around out there, but I haven’t seen any other color in… at least a decade or so in ERs.