What does the blue and red in the ultrasound mean?

Blood Flow, Motion: Doppler detects movement, typically of the blood. Red usually means flow towards transducer, blue away but they can be reversed. Movement of the heart and the cardiac valves can also generate color signals. Jets of urine entering the bladder or amniotic fluid in and out of the fetal nose will also show color.

Vascular flow direct: Assigning color red and blue distinguishes the direction of vascular flow in relation to transducer. The assigned colors are random, but is best to standardize: flow from left side of image to right side of image is assigned red; opposite flow assigned blue.