The source code of voxl-send-neopixel-cmd test tool is here : https://gitlab.com/voxl-public/voxl-sdk/services/voxl-io-server/-/blob/master/tools/voxl-send-neopixel-cmd.c
The function that creates a binary packet (which is forwarded to the ESC via PX4) is here : https://gitlab.com/voxl-public/voxl-sdk/services/voxl-io-server/-/blob/master/lib/modal_io.c#L181 . Basically you need to provide the following information to this function:
number of LEDs you are controlling LED type (RGB or RGBW) array of LED colors (3 bytes for each LED in case of RGB, or 4 bytes per LED for RGBW). the array should contain all LED values ranging from 0-255, so if you had 3 LEDs and first should be red, second green and third blue (50% power), the LED color array would be : [127, 0, 0, 0, 127, 0, 0,0,127]. this function will create a packet with checksum and send it to PX4, and PX4 will forward the packet to the ESC.You can modify the voxl-send-neopixel-cmd to do what you need, maybe you can make it accept an array of LED color values via command line.
I know the tool could have been more helpful. If you describe your use case, maybe I can help improve the voxl-send-neopixel-cmd tool.
Alex