Doppler Information
The colors are the different values of energy that are reflected back toward the radar. Called echoes, the reflected intensities are measured in dBZ (decibels of z). As the strength of the signal returned to the radar increases the dBZ values increases. The Doppler radar does not determine where rain is located, only areas of returned energy.
The "dB" in the dBz scale is logarithmic and has no numerical value, but is used only to express a ratio. The "z" is the ratio of the density of water drops (measured in millimeters, raised to the 6th power) in each cubic meter (mm^6/m^3). Mathematically: dBz= 10 * log (z/z0) Where z = reflectivity factor and Z0 is defined to be 1 mm^6/m^3. When the "z" is large (many drops in a cubic meter), the reflected power is large. A small "z" means little returned energy. In fact, "z" can be less than 1 mm^6/m^3 and since it is logarithmic, dBz values will become negative, as often in the case when the radar is in clear air mode and indicated by earth tone colors.
The scale of dBZ values is also related to the intensity of rainfall. Typically, light rain is occurring when the dBZ value reaches 20. The higher the dBZ, the stronger the rain rate. One question that comes up is why display any DBZ values below 20 if it doesn't represent rain. The answer is that in severe storm areas the lower DBZ values help identify Outflow Boundary which can help in storm path predictions.
More information here: http://forecast.weather.gov/jetstream/doppler/baserefl.htm