To determine the direction of blood flow, a color bar displays the corresponding directional colors. Colors near the baseline have lower velocities, while higher velocity blood flow displays at the ends of the color bar.

Using the color bar as a guide, you can see that very low velocity blood flow moving toward the transducer appears in dark red, close to the baseline. The higher velocity flow has an assigned color further up in the color bar into the orange range. The highest velocity blood flow moving toward the transducer is yellow. Conversely, the lower velocity blood flow moving away from the transducer is dark blue; the higher is light blue.

Notice that no numbers correspond to the colors. Yellow represents a higher velocity blood flow than orange, because it is higher up the color bar. How fast is yellow? How much faster is yellow than orange? The answers reveal an important difference between conventional Doppler and color flow imaging.