I ended up doing a B/W scan at a lower resolution (my lawyer said it would be fine) and the document shipped okay - and before your post. However, I did run a test using your solution and, yes, that colour JPG would have shipped just fine. Thanks for the tip.
Also if you use Photos to "send to email" a picture, it will always give you several options on what size of image to send, can scale it down to a reasonable size for you automatically on-the-fly.
