If iMessage is switched off either on your iPhone or on the recipient's iPhone, the message will be sent via SMS and due to this, the message background turned into green color. It might be also the case that the iMessage server is down for temporary either on your iPhone or recipient's iPhone.
If your iPhone messages are green, it means they're being sent as SMS texts rather than iMessages. You'll always see green when texting Android users, or when you're not connected to the internet. If all of your iPhone messages are green, you should make sure iMessage is turned on in Settings.
iMessages only work between Apple users. You'll always see green when writing to Android users, or when you're not connected to the Internet (How To Transfer Messages To New iPhone).
The Recipient Wasn't Using an Apple Device at the Time
Remember, iMessage only works between Apple devices. So, if your family or friend switches from an iPhone to an Android smartphone, for example, your iMessage will automatically be sent as a text message to them.
If the chat bubble is green, that could indicate someone has blocked you, especially if your chats with this person used to be blue. However, this is only true if both of you use iMessage on iPhones. If the other person switched to an Android phone, their bubble may have turned green (plus, they may have a new number).
If the notification “iMessage Sent as Text Message” only appears occasionally, it may simply be due to poor internet connectivity. However, if this happens consistently with one particular contact, there may be a chance that you have been blocked by them.
Since Android and other carriers operate within the SMS and MMS standards of text communication, these green texts are more versatile in who they can reach with their texts. Blue messages, or iMessage messages, are compatible within the Apple ecosystem.
Green messages are SMS, Blue messages are iMessage. iMessages can only be sent when both parties have an Apple Device, have enabled iMessage, and have internet access. If either party loses internet access, then the Messages App will default to text messages (carrier based SMS) to try send the message.
Green texts, on the other hand, move text around the world over a basic cellular signal, like a flip phone of yore. What this means is that if you've exceeded your phone plan's data limit, or you're out of range of LTE or Wi-Fi, iMessage won't work. Your phone will likely resend the text as a green bubble SMS.
If it's green, it's an ordinary text message and doesn't offer the read/delivered receipts.
Go into Settings on your phone, go to Messages and make sure the first option, "iMessage" is still ticked.
Because it is known to cause suspicion of cheating the solution for many cheaters is to turn off their notifications or hide them to avoid questions.
So if the person's iMessage is off, and you try to send it, there name will not appear in blue and a normal carrier text message will go.
Texting someone who's blocked you works as you would expect. The message sends as normal, and you don't get an error message. This is no help at all for clues. If you have an iPhone and try to send an iMessage to someone who has blocked you, it will remain blue (which means it's still an iMessage).
Go to Settings>Messages and make sure that iMessage is activated. iMessage will only send to other iOS devices. The iPhone will send SMS if you are sending to non-iOS devices. You also need to make sure that you have data turned on, and be either connected to Wi-Fi, or cellular data.
“The simplest way to tell if you have been blocked by an Android user is to call,” Lavelle says. Just like with an iPhone, listen for it to be diverted to voicemail or play you a pre-recorded message.
SMS is available on virtually every phone and tablet in the world. iMessage on the other hand is available only on Apple devices. But one of the major differences and in fact, the most important is security. iMessage texts contain a thing called end-to-end encryption that regular SMS texts don't.
Blocked text messages disappear
When someone that you've blocked texts you, their texts go nowhere. They won't receive any notification that they're blocked, and their messages will still look like they've been sent. But you won't receive any of them, and you can't respond.
If you send a message as a regular SMS in the green text bubble and you do not get a “Delivered” notification but instead get a notification similar to “Message not Delivered” or no message at all, that means you were probably blocked.
If you call someone that hasn't blocked your number, you typically hear several rings before it goes to voicemail. When you call a number that has blocked yours, you may hear one or half a ring or no rings at all and then the call will go to voicemail.