This is normal. When the DLS 2 starts up, it attempts to calibrate, which requires it to be still and motionless. During this time, the LED flashes rainbow colors. Ensure that there is no vibration or movement until the DLS 2 has completed this procedure, indicated by normal LED status lights (shown in the User Guide).
Occasionally, the drone's fan can vibrate the DLS 2 during the calibration, causing it to stay in that mode for some time. Users can apply light pressure with a thumb and finger to stabilize the DLS 2 and allow it to finish calibration. If that still does not work, users can tilt the aircraft/DLS until the DLS rests on the ground to attempt to stabilize the vibrations. Make sure your firmware version is up-to-date, as more recent versions help minimize the "rainbow mode" problem.