Typically, when damage occurs to an LED Light Panel, it is only partly lit. Flexing the panel will damage the circuit board, causing dark or miscolored areas depending on the panel type. The damage can be hard to detect at the time of installation, but it will worsen over time. It is always important to handle an LED Light Panel …
The Vivid Pixel fixture will turn a solid blue when it is receiving power, but not data. What if blue dots illuminate on the fixture? This occurs when the connection of data is to the signal output side instead of the signal input side.
Is the unlit cutting unit at an end? The two causes of this are improper assembly or improper cutting. Rather, is the unlit cutting unit in the middle of the run? Improper twisting or bending is likely the cause. This would cause damage to the LEDs as well as the circuit board.
The cause of random colors could be either connection problems, or shorts along the fixture. Try connecting the fixture directly to power to see if the issue could be the decoder. Also check for resistance along the circuit board.