Color Doppler imaging is most effective when the angle between the ultrasound beam and the flow of blood is small.
At an angle of 88 degrees, the flow of blood is nearly perpendicular to the ultrasound beam.
When the angle is close to 90 degrees, the Doppler shift (frequency change) approaches zero, resulting in little to no color being visualized on the Doppler image.
Thus, to obtain a color signal, the angle should be optimized to be as close to 0 degrees as possible, with 60 degrees being the practical limit for accurate Doppler measurements. Reference:
ARDMS Sonography Principles and Instrumentation guidelines on Doppler angle and its effect on Doppler imaging.