Here is Tom Moffat's explanation... It seems that that doppler radar device is one radar which can be programmed for several different purposes. The radar itself simply collects three basic kinds of data: reflectivity, mean axial velocity, and spectral response. Reflectivity can come from from clear air or clouds or storms, as I mentioned earlier. Mean radial velocity is what the "doppler" part of the radar is for; it's a measure of how fast a reflecting object is moving toward or away from the radar. If the object is moving semi- crossways in relation to the radar, only the component directly to or from the radar is detected. However the instantaneous direction the radar is pointing can be used in conjunction with mean radial velocity to work out how fast an object is moving in ANY direction.
The spectral response indicates what wavelength of radar energy is being best reflected. It gives an indication of what the reflecting body is made of (clear air, water vapour, clouds, rain, etc.).
It appears the doppler radar we are getting pictures from can use the above measurements, processed through a computer, to operate in several different modes, among them "clear air" and "precipitation". On a clear day it strarts off in clear air mode, but the first rain it encounters causes it to automatically switch to precipitation mode.
In the upper right-hand part of the radar display is a mode indicator; this afternoon it was saying "precipitation mode". And to the south of Seattle was a long line of what appeared to be some most unpleasant storms. I'll bet during those clear days last week it said "clear air mode" and was busy looking for things like clear air turbulance and wind shear.
I haven't had the pleasure of seeing the doppler radar display in color (due to my mono LCD laptop screen) but I would bet that the colors displayed range from blues and greens for less intense effects, up through yellows and oranges and reds and pinks up to white for really severe storms. The colors are "false colors" used to represent things like intensities or temperatures. The same color scheme is used pretty much all over the world, and it was very common with the weather satellite imagery stuff I was working with in Australia.