Assuming that the instrument’s parameters are appropriately set for the range of velocities to be depicted in color when superimposed upon the gray-scale image, RED means flow in one direction while BLUE means flow in the opposite direction. A mixture of RED & BLUE together can be the result of circular flow, turbulence, aliasing, or insufficiently resolved coherent flow.
Power Doppler is a form that is more sensitive to slow flow, but is depicted as one color, e.g. gold, orange, yellow to indicate only its presence but no information as to which direction it is, if any.
There are other forms of Ultrasound Imaging which employ color to connote, for instance, the use of harmonics. Such color assignments are proprietary, i.e. depend on the manufacturer.