This can look complicated, but there's some mathematical logic behind it. It's basically this way:
Flash gets the color value for each channel. Let's suppose this is R (Red), and its value is 255 (full red).
Flash checks the percentage value and calculates the new R value based on this value. In this example, the R value is 100%, that is, normal value. 100% of 255 is still 255, so it'll continue to be 255.
Flash then adds (or subtract) the value on the "adding" field. In this example the "adding" field value is zero so the color value will continue to be 255.
Continue with the other color channels - G (Green), B (Blue) and Alpha, the transparency for the object.
As you can see, in this example all the channels will continue to be the same, because the dialog window is at its default values. But what if we play a bit with this settings?