Color aliasing in Doppler ultrasound occurs when the velocity of blood flow exceeds the Nyquist limit, causing the color display to wrap around and display high velocities incorrectly as the opposite direction. This phenomenon is characterized by a mix of colors that indicate flow in both directions at the same location. In the provided image, there is a clear presence of color aliasing, as evidenced by the abrupt color change across the vessel, which is not consistent with normal flow patterns.