Ok, I think I now know enough to start 'playing' with the Color Editor, but it seems a lot like a different kind of 'trial and error'. For example,
1. Picking the right gammagroup is still a guess when the name does not obviously relate to the element it is for.
2. FAQ 9 says: "Why is the resulting color for the graphical element not the color that I see above each slider?
The color you see above each slider is the color you get for a pixel that has the “Preview source color” values for its color channels. To get an accurate result, you should select a previous source color that is approximately the average color of the graphical element you are tweaking. For instance, if you are changing a graphical element that has only red (ie: it has 0 in its green and blue channels), no matter the poxition of the green and blue sliders, it will not change anything in the element. If that happens, you can use one of the two grayscale checkboxes, so that the element will be grayscaled before the color tweak is applied, or you can use the saturate channels checkbox to boost its color channels so that all three sliders will have an effect."
The gammagroups have gray (grayscale) and boost (saturate) values (i.e. 0, 1, or 2) and the Color Editor has checkboxes for max grayscale, avg grayscale, and saturate channels. There is no explanation of the correlation between the values and the checkboxes.
But at least there is an immediate indication of what the color may be after changing the red, green, and blue values and whether a gray and/or boost value will help achieve the desired color.
3. FAQ 11 says: "Why does the red green and blue values for each color group goes from -4096 to +4096 rather than from 0 to 255?
The red green and blue values in the xml for a Color Theme are not a direct representation of the color you get (otherwise, it would make all pixels this value!), they are (arbitrarilly ranged) values that are used to multiply (or divide) each pixels's channel."
I've read this over and over and I still don't know what it means. Negative values arbitrarily multiply each pixels' channel and positive values arbitrarily divides, or the other way around. Whatever, it's still 'Greek' to me.