I have a four color screen printing machine. When I did the Estes Astro decals for Blackshire, I mixed the orange color by hand, ran a sample, then compared to a picture of the original decals someone had uploaded here. Doing the color that way is kinda hit or miss depending on the monitor, camera used and age of the original decals. I am sure that for 90% of the builders it would be good enough, But you always run into those that are like,
"that should have a red content of 254, not 255"
I mean look at all the controversy over minor points over the years, especially about fins in the era of fin templates. I really want to know if anyone making an Alpha back in the sixties added a sanding kerf to the outline traced on the balsa. And of course if you sanded just a sixty-fourth of an inch too much compared to the template, you threw that fin out and started anew.
Sorry, didn't mean to go off on a rant
The problem is that often our perception of a certain hue CAN differ to a large extent from the original and adding a color swath to even compare a difference from can be helpful in getting a close match.