I'll try to give you the Cliff's Notes. Although, I have to say up front, it's no substitute for attending one of Dru's workshops. I REALLY highly recommend attending one if at all possible. He's as good of a teacher as he is an artist.
That said, color buffering is basically using a white "base", and mixing in transparent colors to match a given color. That is why the Illustration colors are transparent. They were designed with Dru to work with his color buffer system.
Just as a quick review, color has three basic values; hue, saturation, and value. Hue is, basically, what color it is (red, blue, oragnge, whatever). Saturation is how pure the pigment is (i.e. is it REALLY blue, or is it a bit dull?). Value is how light or dark the color is. If you look at a black and white picture, you're seeing purely the color values.
Using a white base as a buffer allows you to "lock in" the value of the color. Like an opaque paint, once it reaches saturation, you could have a coat of paint an inch thick or a 1/4 mil thick, and it will be the same. The reason why that is an advantage is that with transparent colors, if you hold the trigger even a fraction of a second too long (or not long enough) the value will be too dark or too light.
The basic method is to isolate a color (start with lighter colors and move to darker ones). The easiest way to do this is to cut a small "window" on a white 3"x 5" note card, and tape it to the reference pic so that the goal color is showing through the window. Then, mix a buffered color match. Mixing "in the cup" isn't recommended here. Have a separate cup, mix the color as close as you can, then put a few drops into your airbrush, and test spray it on the edge of a white piece of paper. Then, compare your color to the isolated color, and adjust as necessary. It will take quite a few tries at first, but you will get better at color matching the more you do it.
After that, indentify the areas on the painting with that color, and apply it where you see it.
For things like feathers, hair, or fur, I'll usually identify the lightest value, and spray a base coat of that down, followed by a coat of transparent base. Then I'll mix up my transparent colors, apply them in light coats, and use my scratching and erasing techniques, being very careful not to scratch through the base coat.
There's a ton more to it that Dru could show you that I can't, but that's the really, really basic description. (of the 4 days of course time, the part I'm describing makes up maybe an hour of the class time, just to give you an idea of how much more there is in the real deal)