Rocket science, no but it isn't as easy as you may think either. Have to actually try it to understand it. I know I did not fully understand it until I tried my own hand at it and I still have a lot more to learn.

Defining what full spectrum is might be the first step. IMO, FS means that there has to be significant representational amounts of each spectral hue in order to qualify for full spectrum. Balance then becomes the challenge, the issue. The game becomes you have to mix a pretty color using all seven spectral hues and you can't use black or gray to soften or change color characteristics. And then... you have to be able to duplicate your process so each batch is a match -- multiple times. Truly, not so easy.

Time and gallons of paint fly by. One drop of one color can blow hours worth of color tinkering. It is very easy to come up with a mud-colored brew. Not so easy to come up with pretty, pleasing, viable wall colors.

The first two obstacles are the base and the colorants. Have to know each intimately and are why a full spectrum formula doesn't just transfer to any can of base from any paint store.

Depending on what colorants/pigments you are using, you have to understand their inherent qualities. For instance, the colorants from ICI are a bit unusual in that their black really isn't black, it's navy. Their green isn't green, it's teal. Each colorant has sub-color signatures (if you will) that must be considered and then dealt with. There is a code of sorts with each, the base and colorants, that you have to figure out first in order to mix forward.

Dumping leftover paint colors together and calling it full spectrum isn't really mixing your own full spectrum color. Don't know exactly what one would call that, but in my book, it does not qualify. If the result is pleasing colors, then maybe I'd just say it works and sometimes that's good enough! lol! :-)

The other thing I've learned spec'ing the different FS brands is that each may seem to be different in their color quality, but I really don't believe they are. Can't make arbitrary, lump sum comments about any of the colors or FS designers. Kind of naive to do that actually, because I've found that the inherent lighting and elements in each individual home/room, defines the quality of particular FS colors.

To say one FS brand is more in some respect than the other is ambiguous. One brand (heck, one color!) may indeed work better for one home, one room, one person than another but that doesn't mean that the other FS brands and colors are less because of it.

After going thru the experience of mixing my own FS colors, I arrived at the conclusion that the FS brands offer a service of value. I certainly have A LOT MORE to learn about it. Where I am right now is that it's more efficient, cheaper, and easier for me to just order it from the sources that currently exist.

Hope that helps some.

I got Donald Kaufman's book on interlibrary loan and he does have two colors available to mix. I'm nervous though. For the reasons that you cited. Colorants from one manufacturer are likely not the same as from another. There was a beige and a grey. I am inclined to perhaps try the grey but I'm torn. Luckily paint is the cheapest of all the ways one can decorate. It's a lot easier to change paint color than carpet or flooring.