Color map

In color Doppler, the color map is used to display the Doppler shifts.

The color map is divided into two channels, positive and negative, separated by a baseline.  The positive channel maps the positive Doppler shifts from blood moving towards the transducer and is ALWAYS on the top.  The negative channel maps the negative shifts from blood moving away from the transducer and is ALWAYS on the bottom.   

Low Doppler shifts are mapped in darker colors close to baseline.  Higher Doppler shifts are mapped in lighter colors farther from the baseline.  The end of each channel is the Nyquist limit.  

A variety of different maps are available.  However, the most common map is the red/blue map.   When non-inverted, this map is called the BART map (blue away, red towards).  Echocardiographers use the BART map, while vascular and other sonographers may invert the color map.   (See inverting the color map, for more details)