Check your iMessage settings. The other party could have iMessages turned off for some reason, or it signed out. If the user is in a phone call, there is no data, so the message will be sent SMS. If the user has data turned off and is not at a wifi access point, the message will be sent SMS.
Thanks! I actually checked with 2 of "green" message people, and their iMessage option had been turned off - neither knew why or when, but they turned iMessage back on, and we are blue again!