The most common Doppler mode you will use is color Doppler. This mode allows you to see the movement of blood movement in arteries and veins with blue and red patterns on the ultrasound screen.
A common question that comes up with color Doppler is: What do the colors on ultrasound mean? The answer is: RED means there is flow TOWARDS the ultrasound probe and BLUE means that there is flow AWAY from the ultrasound probe. It is a misconception that red is arterial and blue is venous. It actually just depends on the direction blood is flowing relative to the angle of your ultrasound beam.
An easy way to remember this is to use the BART mnemonic: Blue AWAY, Red TOWARDS.
There is a mode similar to color Doppler that you may encounter called Power Doppler. This mode does not show up as red or blue on the screen but only uses a single yellow color signifying the amplitude of flow. It is more sensitive than color Doppler and is used to detect low flow states such as venous flow in the thyroid or testicles.