I’ve found that in the one project I did something similar, it would usually turn on at boot and I had to shut it off. Ended up adding this to the boot sequence (and still some times it does this). Seemed to be a common problem, if I remember correctly.
I tried another LED, end up that the soldering was bad.
I can now turn on / off the light using NeoPixel Light.
Can you copy the code that you are using at boot to set the color ?
I repeated it 3 times with delays because I still had occasional times where it wouldn’t shutoff (and even with this, there are times that it stays on). It’s always still controllable though. If I remember correctly, I came across other instances of folks having similar issues. I suspect there is some noise that’s getting on the data line at boot, but don’t know for sure and since I already made up a PCB for this particular project, troubleshooting was more effort than it was worth.