This seems to be a bit of a split thread. Some are trying to send only as SMS while others are trying to get their iMessage to send back and forth once again only with blue bubble, and no SMS (green).
I have discovered a fix for the latter (iMessage only - blue). Seems it has nothing to do with the settings on YOUR phone, but it is related to what is incoming (the settings on the INCOMING PHONE) that will depict the type that you receive (green) and how it replies (stays green if incoming was green.)
My brother and I both have iPhones, but one day a while back, he was not getting reliable cellular data (couldn't open websites etc) and while he was trying to iMessage me, his phone prompted him to send as SMS, and he accepted. When that message came to me, it was green. And after that point in the message stream, all my replies, and all further messages from him were green or SMS.
This morning, I asked him to check Settings>Messages> then check if iMessage was toggled on or off. He said his was off, but he didn't know how it got turned off. This was probably an automatic toggle when the phone prompted him to "send as SMS". So he toggled it back on.
So, I sent him a new message, and was frustrated to find that the bubble was still green-SMS! But... when he replied, it came back as blue-iMessage! And all further messages are now iMessage...
Hope our experience helps someone else.
Cheers