Pure white is the highest, absense of all colors (black) is the lowest. You can alway set the bump depth to negative if you make it the other way around by accident.

Its like the mesh is a cloth and the cloth is like layed over a pattern. Its like pulling a cloth over dirt, you get the dirt's bumpiness, but the cloth's color,specularity, ect. This is basically how I see it anyway. The images can have color too, because the bump reads the Value, in the HSV (or HSB, Brightness), of the pixels. This is the dark-light bar in the color picker.