If I remember rightly these were supposed to be used to adjust the idle mixture setting of your carburettor as you needed to be able to see the colour of the flame through the clear insulator of these spark plug substitutes while making the adjustment & reading the chart to make sure that you got it right.

It's very difficult to look at them while the car is being driven, & reving up the engine does not substitute for driving with a load on the engine, so idle mixture tuning was a practical limit for them.
They were not intended to be used as 'in service' spark plugs either as they did not come in a variety of heat ranges.

With the industry wide shift to EFI [no new cars with carbs to adjust] & the novelty value of these gimmicks wearing off, sales fell off & manufacture came to an end.

Many people discovered that the idle mixture adjustment could be done just as well with a good old vacuum guage &/or a tachometer which were much easier to fit & use than changing a spark plug twice. [remove spark plug & fit colourtune, then remove it & refit a real plug.] Oh yeah, they were also difficult to use if you were doing a tune up in your driveway in the strong sunlight of a lovely summers day, so you needed someplace well shaded to do it, or better yet, do it at night to get a real good look at the colour of the combustion.

Good fortune to you in locating one.