From: National Weather Service Jet Stream - Online School for Weather

When the motion of storms are subtracted from the wind field, the result is a
picture of the wind as if the storms were stationary. As in the case of all
velocity images, red color indicates wind moving away from the radar with green
color indicates motion toward the radar. Small scale circulations, from which
tornadoes often form, will typically be indicated by strong inbound wind located
beside strong outbound wind relative to the radar.