wildpikey wrote: I've done a lot of research and cannot find an answer I'm happy with.
No opinions please (that's why I posted here rather than Thai Visa) only if you actually know for sure.
Well you are making it rather difficult for people to reply because there will be a number of opinions and nobody really "knows for sure".
That is because the term has been used on various forums by different people many of whom will have had different experiences and ideas about their grey books.
However I will have a go, to start things off and then no doubt other people will add their comments or corrections.
When you buy a new bike from an official outlet in Thailand, you will recieve, usually about a month after the purchase a Green covered book for the bike. This lists the bike details, and your details as purchaser. Somebody else later buys the bikefrom you and the local transport office will with the correct back up paperwork, insert the buyers name as second owner of the bike and so on. This is the official book and certainly not a Grey one.
Bikes which have arrived in Thailand by other means than the official distributor, and many of these have come as bike parts and the reduced duty for parts paid and noted on an invoice are assembled by local shops and sold as complete bikes. These bikes have no books, and are sometimes called bikes with invoice only. Some dealers, for a lot of money (50,000 up) will offer what is probably a Grey Book, this is still a green covered book, probably belonging to a different , old bike from the same manufacturer, which had the same cc. engine. As the frame and engine numbers in this book will not agree with those on the bike, the ones on the bike are ground off and the bike restamped with the numbers in the book. These books can take 3 months or more to arrive. Depending on which province you try to later sell or register this bike and the attitude of the local staff, you may have problems when trying to change ownership. With this type of book as long as you do not try to sell the bike, you will probably have no problem getting annual tax and insurance coverage.
Books from Prachinburi and Rayong provinces are more likely to be grey than some other provinces.