When I go to send a text message on my Apple Watch, whether it be through Siri or the messages app, it always tries to send a text message (green) rather than a iMessage (blue). I have tried turning on and off iMessage, restarting my iPhone and Watch, unpaired and re-paired my watch, and setting it up again as new. My watch will appear to try to send a regular (green) text message through my phone, but will get stuck on "Sending..." even though the text actually gets sent. I can send iMessages just fine on my iPhone. The people in my contacts who I am messaging do have iPhones and their contacts are set up as having an iPhone.
And a little while later, my watch will say that the message failed to send, even though they were sent:
As you can see below, texts from my Watch end up sent as regular text messages, while texts from my iPhone to the same person get sent as iMessages. In the end, how do I get my watch to send iMessages?
I had an Apple Store Genius appointment a while back to try and resolve this. At the time I was running the iOS 8.4 public beta and I was told to wait for final release of iOS 8.4 to fix my problem, as the Apple employee was also experiencing the same problem and was also on the beta. Now with the final release of iOS 8.4 out, I've updated and it still doesn't work. I'm also updated to Watch OS 1.0.1.
My Watch will also occasionally tell me that my iCloud password is "incorrect" and that I need to sign into iMessage/FaceTime again (my password isn't wrong, I've done this multiple times). Although it seems to only ask me this when the Watch dies or is manually restarted.