I thought I remembered that the problem was the number wasn't long enough, but it looks like it is 12 digits just like Colorado asks for. Maybe it was something else, order or letters or spaces or ??? I do remember having to drive the boat out after my initial attempt at registration was denied and have the park person get frustrated, but I can't offhand now see why. Thanks for being attentive to this among many other issues.
Colorado gives the option of in-person registration at any state park, which I think makes non-standard (like rafts) easier, since it's harder to argue with the existence of something that is right in front of you.
And, off topic, but why isn't white a stock color choice?