Why is it that there were only red and blue dots on one of my kidneys and not the other in an ultrasound?

May be...: The presence of red or blue dots on Doppler ultrasound study merely gives a rough indirect saying if arterial or venous blood there; they may not say much about how important they are. But, if a mass is suspected, need to further define its nature with either CT or MRI (usually CT), if clinical condition deemed necessary. e.g., evaluating blood in the urine. For individual need, ask Doc timely.

False color-chk: The red & blue dots r most likely the false color signals from the Doppler mode of the ultrasound (US) machine. Doppler US helps determine blood flow; however, it is very dependent on how the transducer ("wand") is held in relation to the body. More likely, Doppler was applied to 1 of ur kidneys, & not to the other b/c the tech or doc thought he/she saw something. Anything abnl shd b in the report