I get it.... my point is, that there can be other outside influences... or even depending on where the sensor 'checks' you can have slightly different values.
So while the same thread is checked '3' times.. you may get '3' slightly varying values.
How does that work for 93 yo relative?
I'm also not clear on how you planned on displaying this 'data' (response) to your relative?
I dont think RGB values will do them any good.
I dont think you can work out a pre-defined map.. as who knows where and when they'll get new 'yarn'..
I still think making an APP in (maybe) MIT App Inventor (or something) might be the bets approach.
You can just have a 1 big button.. that says SCAN for the relative to interact with.